Documentation: git-clean: make description more readable

The existing text is a little bit awkward. This rewrites the description
section to be more readable and friendly.

Signed-off-by: Wesley J. Landaker <wjl@icecavern.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: wjl

Documentation/git-clean.txt

@@ -12,14 +12,17 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
-Removes files unknown to git.  This allows cleaning the working tree
-of files that are not under version control.  If the '-x' option is
-specified, ignored files are also removed, allowing the removal of all
-build products.
+
+This allows cleaning the working tree by removing files that are not
+under version control.
+
+Normally, only files unknown to git are removed, but if the '-x'
+option is specified, ignored files are also removed. This can, for
+example, be useful to remove all build products.
+
 If any optional `<path>...` arguments are given, only those paths
 are affected.
 
-
 OPTIONS
 -------
 -d::