[Clang] Clarify __builtin_memcpy_inline documentation

[gchatelet]

Summary

This patch updates the documentation about __builtin_memcpy_inline and reorders the sections so it is more consitent and understandable.

[courbet]

clang/docs/LanguageExtensions.rst

2439

why capitalize ?

[gchatelet]

clang/docs/LanguageExtensions.rst

2439

I somewhat missed this comment, I'll fix this as a separate commit.

[gchatelet]

clang/docs/LanguageExtensions.rst

2439

Done in rGb8ea47a

clang/docs/LanguageExtensions.rst

 despite these functions accepting an argument of type ``const void*``.
 
 Support for constant expression evaluation for the above builtins can be detected
 with ``__has_feature(cxx_constexpr_string_builtins)``.
 
 Memory builtins
 ---------------
 
- * ``__builtin_memcpy_inline``
-
-.. code-block:: c
-
-  void __builtin_memcpy_inline(void *dst, const void *src, size_t size);
-
-``__builtin_memcpy_inline(dst, src, size)`` is identical to
-``__builtin_memcpy(dst, src, size)`` except that the generated code is
-guaranteed not to call any external functions. See LLVM IR `llvm.memcpy.inline
-<https://llvm.org/docs/LangRef.html#llvm-memcpy-inline-intrinsic>`_ Intrinsic 
-for more information.
-
-Note that the `size` argument must be a compile time constant.
-
 Clang provides constant expression evaluation support for builtin forms of the
 following functions from the C standard library headers
 ``<string.h>`` and ``<wchar.h>``:
 
 * ``memcpy``
 * ``memmove``
 * ``wmemcpy``
 * ``wmemmove``
 
 In each case, the builtin form has the name of the C library function prefixed
 by ``__builtin_``.
 
 Constant evaluation support is only provided when the source and destination
 are pointers to arrays with the same trivially copyable element type, and the
 given size is an exact multiple of the element size that is no greater than
 the number of elements accessible through the source and destination operands.
 
-Constant evaluation support is not yet provided for ``__builtin_memcpy_inline``.
+Guaranteed inlined copy
+^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code-block:: c
+
+  void __builtin_memcpy_inline(void *dst, const void *src, size_t size);
+
+
+``__builtin_memcpy_inline`` has been designed as a building block for efficient
+``memcpy`` implementations. It is identical to ``__builtin_memcpy`` but also
+guarantees not to call any external functions. See LLVM IR `llvm.memcpy.inline
+<https://llvm.org/docs/LangRef.html#llvm-memcpy-inline-intrinsic>`_ Intrinsic 
+for more information.
+
+This is useful to implement a custom version of ``memcpy``, implemement a
+``libc`` memcpy or work around the absence of a ``libc``.
+
+Note that the `size` argument must be a compile time constant.
+
+Note that this intrinsic cannot yet be called in a ``constexpr`` context.
+
 
 Atomic Min/Max builtins with memory ordering
 --------------------------------------------
 
 There are two atomic builtins with min/max in-memory comparison and swap.
 The syntax and semantics are similar to GCC-compatible __atomic_* builtins.
 
 * ``__atomic_fetch_min``