[TableGen] Clean up documentation toctrees; clarify two paragraphs.

[Paul-C-Anagnostopoulos]

Summary

This patch cleans up the TableGen documentation toctrees.

It also clarifies two paragraphs in the Backend Guide and adds a cross-reference.

I will auto-LGTM this in 24 hours.

[Paul-C-Anagnostopoulos]

I will push this now.

llvm/docs/TableGen/BackGuide.rst

 This document is a guide to writing a backend for TableGen. It is not a
 complete reference manual, but rather a guide to using the facilities
 provided by TableGen for the backends. For a complete reference to the
 various data structures and functions involved, see the primary TableGen
 header file (``record.h``) and/or the Doxygen documentation.
 
 This document assumes that you have read the :doc:`TableGen Programmer's
 Reference <./ProgRef>`, which provides a detailed reference for coding
-TableGen source files. This document and the data structure comments will be
-improved over time.
+TableGen source files. For a description of the existing backends, see
+:doc:`TableGen BackEnds <./BackEnds>`.
 
 Data Structures
 ===============
 
 The following sections describe the data structures that contain the classes
 and records that are collected from the TableGen source files by the
 TableGen parser. Note that the term *class* refers to an abstract record
 class, while the term *record* refers to a concrete record.

 
 TableGen provides some tools to aid in debugging backends.
 
 The ``PrintRecords`` Backend
 ----------------------------
 
 The TableGen command option ``--print-records`` invokes a simple backend
 that prints all the classes and records defined in the source files. This is
-the default backend option. The output looks like this:
+the default backend option. The format of the output is guaranteed to be
+constant over time, so that the output can be compared in tests. The output
+looks like this:
 
 .. code-block:: text
 
   ------------- Classes -----------------
   ...
   class XEntry<string XEntry:str = ?, int XEntry:val1 = ?> { // XBase
     string Str = XEntry:str;
     bits<8> Val1 = { !cast<bits<8>>(XEntry:val1){7}, ... };

 fields. Note that anonymous records are named ``anonymous_0``,
 ``anonymous_1``, etc.
 
 The ``PrintDetailedRecords`` Backend
 ------------------------------------
 
 The TableGen command option ``--print-detailed-records`` invokes a backend
 that prints all the global variables, classes, and records defined in the
-source files. The output looks like this.
+source files. The format of the output is *not* guaranteed to be constant
+over time. The output looks like this.
 
 .. code-block:: text
 
   DETAILED RECORDS for file llvm-project\llvm\lib\target\arc\arc.td
   
   -------------------- Global Variables (5) --------------------
   
   AMDGPUBufferIntrinsics = [int_amdgcn_buffer_load_format, ...

llvm/docs/TableGen/ProgRef.rst

 ===============================
 TableGen Programmer's Reference
 ===============================
 
 .. sectnum::
 
 .. contents::
    :local:
 
-.. toctree::
-   :hidden:
-
-   BackEnds
-   BackGuide
-
 Introduction
 ============
 
 The purpose of TableGen is to generate complex output files based on
 information from source files that are significantly easier to code than the
 output files would be, and also easier to maintain and modify over time. The
 information is coded in a declarative style involving classes and records,
 which are then processed by TableGen. The internalized records are passed on