[docs][llvm-nm] Fix documentation regarding llvm-nm reading stdin

llvm-nm reads a.out NOT stdin when no input file is specified. This
patch fixes the doc accordingly, and rephrases the surrounding sentence
slightly.

Reviewed by: grimar

Differential Revision: https://reviews.llvm.org/D63135

llvm-svn: 363065
diff --git a/llvm/docs/CommandGuide/llvm-nm.rst b/llvm/docs/CommandGuide/llvm-nm.rst
index 2a2f827..f679a04 100644
--- a/llvm/docs/CommandGuide/llvm-nm.rst
+++ b/llvm/docs/CommandGuide/llvm-nm.rst
@@ -12,8 +12,9 @@
 The :program:`llvm-nm` utility lists the names of symbols from the LLVM bitcode
 files, object files, or :program:`ar` archives containing them, named on the
 command line.  Each symbol is listed along with some simple information about
-its provenance.  If no file name is specified, or *-* is used as a file name,
-:program:`llvm-nm` will process a file on its standard input stream.
+its provenance.  If no filename is specified, *a.out* is used as the input.
+If *-* is used as a filename, :program:`llvm-nm` will read a file from its
+standard input stream.
 
 :program:`llvm-nm`'s default output format is the traditional BSD :program:`nm`
 output format.  Each such output record consists of an (optional) 8-digit