[OPENMP] [DOCS] fix section formatting issues [NFC]

[kkwli0]

Summary

The section format in HTML shows that the Cuda device section an empty section. This patch is to fix the mismatch. Since the 'Collapsed loop nest counter' is for all platforms, the section is moved under the 'General improvement, section.

[ABataev]

LG

clang/docs/OpenMPSupport.rst

 .. raw:: html
 
   <style type="text/css">
     .none { background-color: #FFCCCC }
     .part { background-color: #FFFF99 }
     .good { background-color: #CCFF99 }
   </style>
 
 .. role:: none
 .. role:: part
 .. role:: good
 
 .. contents::
    :local:
 
-==================
 OpenMP Support
-==================
+==============
 
 Clang fully supports OpenMP 4.5. Clang supports offloading to X86_64, AArch64,
 PPC64[LE] and has `basic support for Cuda devices`_.
 
 * #pragma omp declare simd: :part:`Partial`.  We support parsing/semantic
   analysis + generation of special attributes for X86 target, but still
   missing the LLVM pass for vectorization.
 
 In addition, the LLVM OpenMP runtime `libomp` supports the OpenMP Tools
 Interface (OMPT) on x86, x86_64, AArch64, and PPC64 on Linux, Windows, and macOS.
 
 For the list of supported features from OpenMP 5.0 see `OpenMP implementation details`_.
 
 General improvements
---------------------
+====================
 - New collapse clause scheme to avoid expensive remainder operations.
   Compute loop index variables after collapsing a loop nest via the
   collapse clause by replacing the expensive remainder operation with
   multiplications and additions.
 
 - The default schedules for the `distribute` and `for` constructs in a
   parallel region and in SPMD mode have changed to ensure coalesced
   accesses. For the `distribute` construct, a static schedule is used
   with a chunk size equal to the number of threads per team (default
   value of threads or as specified by the `thread_limit` clause if
   present). For the `for` construct, the schedule is static with chunk
   size of one.
 
 - Simplified SPMD code generation for `distribute parallel for` when
   the new default schedules are applicable.
 
+- When using the collapse clause on a loop nest the default behavior
+  is to automatically extend the representation of the loop counter to
+  64 bits for the cases where the sizes of the collapsed loops are not
+  known at compile time. To prevent this conservative choice and use
+  at most 32 bits, compile your program with the
+  `-fopenmp-optimistic-collapse`.
+
 .. _basic support for Cuda devices:
 
 Cuda devices support
 ====================
 
 Directives execution modes
 --------------------------
 

 Clang supports two data-sharing models for Cuda devices: `Generic` and `Cuda`
 modes. The default mode is `Generic`. `Cuda` mode can give an additional
 performance and can be activated using the `-fopenmp-cuda-mode` flag. In
 `Generic` mode all local variables that can be shared in the parallel regions
 are stored in the global memory. In `Cuda` mode local variables are not shared
 between the threads and it is user responsibility to share the required data
 between the threads in the parallel regions.
 
-Collapsed loop nest counter
----------------------------
-
-When using the collapse clause on a loop nest the default behavior is to
-automatically extend the representation of the loop counter to 64 bits for
-the cases where the sizes of the collapsed loops are not known at compile
-time. To prevent this conservative choice and use at most 32 bits,
-compile your program with the `-fopenmp-optimistic-collapse`.
-
 
 Features not supported or with limited support for Cuda devices
 ---------------------------------------------------------------
 
 - Cancellation constructs are not supported.
 
 - Doacross loop nest is not supported.
 
- Debug information for OpenMP target regions is supported, but sometimes it may
   be required to manually specify the address class of the inspected variables.
   In some cases the local variables are actually allocated in the global memory,
   but the debug info may be not aware of it.
 
 
 .. _OpenMP implementation details:
 
 OpenMP 5.0 Implementation Details
----------------------------------
+=================================
 
 The following table provides a quick overview over various OpenMP 5.0 features
 and their implementation status. Please contact *openmp-dev* at
 *lists.llvm.org* for more information or if you want to help with the
 implementation.
 
 +------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
 |Category                      | Feature                                                      | Status                   | Reviews                                                               |