[docs] Mention gotcha regarding implicit BB numbering

Impetus for the clarification by Mikael Lyngvig.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195812 91177308-0d34-0410-b5e6-96231b3b80d8

docs/LangRef.rst

@@ -128,7 +128,9 @@ lexical features of LLVM:
 #. Unnamed temporaries are created when the result of a computation is
    not assigned to a named value.
 #. Unnamed temporaries are numbered sequentially (using a per-function
-   incrementing counter, starting with 0).
+   incrementing counter, starting with 0). Note that basic blocks are
+   included in this numbering. For example, if the entry basic block is not
+   given a label name, then it will get number 0.
 
 It also shows a convention that we follow in this document. When
 demonstrating instructions, we will follow an instruction with a comment