[docs] Add PR to Lexicon

Summary:
The acronym PR could be ambiguous to some users, especially those who
are used to interpreting it as GitHub's "pull request".

Reviewers: ddunbar, jordan_rose, void, beanz

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25331

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@283465 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed