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

llvm-svn: 283465

llvm-svn: 239085

llvm-svn: 237840

llvm-svn: 217292

llvm-svn: 212977

llvm-svn: 211808

llvm-svn: 175077

Before we learned about :doc:, we used :ref: and put a dummy link at the
top of each page. Don't do that anymore.

This fixes PR14891 as a special case.

llvm-svn: 172162

llvm-svn: 169928

llvm-svn: 165879

llvm-svn: 158813