[dsymutil][doc] Improve wording in manpage and rename file.

 - Improve wording
 - Rename llvm-dsymutil to dsymutil
 - Name -arch=<arch> argument

Differential revision: https://reviews.llvm.org/D39561

llvm-svn: 317226
diff --git a/llvm/docs/CommandGuide/index.rst b/llvm/docs/CommandGuide/index.rst
index a706ba1..805df00 100644
--- a/llvm/docs/CommandGuide/index.rst
+++ b/llvm/docs/CommandGuide/index.rst
@@ -30,7 +30,7 @@
    llvm-stress
    llvm-symbolizer
    llvm-dwarfdump
-   llvm-dsymutil
+   dsymutil
 
 Debugging Tools
 ~~~~~~~~~~~~~~~