[FileCheck] regexp doc update/fix

Minor doc update that the FileCheck matcher supports POSIX ERE.
It also fixes a minor issue in the regexp describing a variable
name: underscores are allowed too.

Differential Revision: https://reviews.llvm.org/D38787



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315679 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed