[docs][llvm-strings] Clarify "printable character" wording

The --bytes option uses the phrase "printable ASCII characters", but the
description section used simply "printable characters". To avoid any
confusion about locale impacts etc, this change adopts the former's
phrasing in both places. It also fixes a minor grammar issue in the
description.

Reviewed by: MaskRay

Differential Revision: https://reviews.llvm.org/D68016

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@372865 91177308-0d34-0410-b5e6-96231b3b80d8

docs/CommandGuide/llvm-strings.rst

@@ -14,8 +14,8 @@ DESCRIPTION
 :program:`llvm-strings` is a tool intended as a drop-in replacement for GNU's
 :program:`strings`, which looks for printable strings in files and writes them
 to the standard output stream. A printable string is any sequence of four (by
-default) or more printable characters. The end of the file, or any other byte
-terminates the current sequence.
+default) or more printable ASCII characters. The end of the file, or any other
+byte, terminates the current sequence.
 
 :program:`llvm-strings` looks for strings in each ``input`` file specified.
 Unlike GNU :program:`strings` it looks in the entire input file, regardless of