document -fpch-instantiate-templates in release notes

[llunak]

Summary

This should have been included in D69585.

[dblaikie]

Looks good (@rsmith might have some nuances in wordsmithing, but if so I think those can be handled in follow-up/post-commit)

clang/docs/ReleaseNotes.rst

 - -ffp-model={precise,strict,fast} provides the user an umbrella option to
   simplify access to the many single purpose floating point options. The default
   setting is ``precise``.
 
 - The default module cache has moved from /tmp to a per-user cache directory.
   By default, this is ~/.cache but on some platforms or installations, this
   might be elsewhere. The -fmodules-cache-path=... flag continues to work.
 
+- -fpch-instantiate-templates tries to instantiate templates already while
+  generating a precompiled header. Such templates do not need to be
+  instantiated every time the precompiled header is used, which saves compile
+  time. This may result in an error during the precompiled header generation
+  if the source header file is not self-contained. This option is enabled
+  by default for clang-cl.
+
 Deprecated Compiler Flags
 -------------------------
 
 The following options are deprecated and ignored. They will be removed in
 future versions of Clang.
 
 - ...