[docs] Add "NFC" to Lexicon.rst.

Author: jrose-apple

docs/Lexicon.rst

@@ -116,6 +116,12 @@ source code, tests, and commit messages. See also the `LLVM lexicon`_.
     version information and may try to load older module files, resulting in
     crashes in ``clang::ASTReader``.
 
+  NFC
+    "No functionality change." Written in commit messages that are intended to
+    have no change on the compiler or library's behavior, though for some this
+    refers to having the *same* implementation and for others merely an
+    *equivalent* one.
+
   open existential
     An `existential` value with its dynamic type pulled out, so that the 
     compiler can do something with it.