[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

docs/Lexicon.rst

@@ -180,6 +180,10 @@ O
 P
 -
 
+**PR**
+    Problem report. A bug filed on `the LLVM Bug Tracking System
+    <http://llvm.org/bugs/enter_bug.cgi>`_.
+
 **PRE**
     Partial Redundancy Elimination