blockfreq: Document BlockFrequencyInfo terminology

Documents terminology used in the forthcoming rewrite of
BlockFrequencyInfo.

<rdar://problem/14292693>

llvm-svn: 206086
diff --git a/llvm/docs/index.rst b/llvm/docs/index.rst
index a55795e..93a1600 100644
--- a/llvm/docs/index.rst
+++ b/llvm/docs/index.rst
@@ -215,6 +215,7 @@
 
    AliasAnalysis
    BitCodeFormat
+   BlockFrequencyTerminology
    BranchWeightMetadata
    Bugpoint
    CodeGenerator
@@ -298,6 +299,10 @@
 :doc:`BranchWeightMetadata`
    Provides information about Branch Prediction Information.
 
+:doc:`BlockFrequencyTerminology`
+   Provides information about terminology used in the ``BlockFrequencyInfo``
+   analysis pass.
+
 :doc:`SegmentedStacks`
    This document describes segmented stacks and how they are used in LLVM.