[base/allocator] Update allocator shim documentation.

We've been using the UCRT symbols since 2016, but the documentation was
never updated.

Change-Id: I2e4f6e7378d1be2d1091b2f0b0f04ce2113ae3c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2404688Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806233}

base/allocator/README.md

@@ -124,6 +124,9 @@ This stage takes care of overriding the symbols `malloc`, `free`,
 allocator shim (next point).
 This is taken care of by the headers in `allocator_shim_override_*`.
 
+*On Windows*: Windows' UCRT (Universal C Runtime) exports weak symbols, that we
+can override in `allocator_shim_override_ucr_symbols_win.h`.
+
 *On Linux/CrOS*: the allocator symbols are defined as exported global symbols
 in `allocator_shim_override_libc_symbols.h` (for `malloc`, `free` and friends)
 and in `allocator_shim_override_cpp_symbols.h` (for `operator new`,
@@ -173,18 +176,6 @@ at build time and ultimately routes the allocator calls to the actual allocator
 headers in `allocator_shim_default_dispatch_to_*` files.
 
 
-Appendixes
-----------
-**How does the Windows shim layer replace the malloc symbols?**
-The mechanism for hooking LIBCMT in Windows is rather tricky.  The core
-problem is that by default, the Windows library does not declare malloc and
-free as weak symbols.  Because of this, they cannot be overridden.  To work
-around this, we start with the LIBCMT.LIB, and manually remove all allocator
-related functions from it using the visual studio library tool.  Once removed,
-we can now link against the library and provide custom versions of the
-allocator related functionality.
-See the script `preb_libc.py` in this folder.
-
 Related links
 -------------
 - [Unified allocator shim doc - Feb 2016][url-allocator-shim]