[clang] Fix isExternC matcher docs

The wording in the documentation for the matcher isExternC 
appears to be misleading since this matcher 
is applicable to functions and variables as well. 
This diff changes the comment and regenerates the html file.

Differential revision: https://reviews.llvm.org/D38151


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@314022 91177308-0d34-0410-b5e6-96231b3b80d8