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

[jhenderson]

Summary

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 formers phrasing in both places. It also fixes a minor grammar issue in the description.

llvm/docs/CommandGuide/llvm-strings.rst

 llvm-strings - print strings
 ============================
 
 .. program:: llvm-strings
 
 SYNOPSIS
 --------
 
 :program:`llvm-strings` [*options*] [*input...*]
 
 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
 file format, rather than restricting the search to certain sections of object
 files. If "``-``" is specified as an ``input``, or no ``input`` is specified,
 the program reads from the standard input stream.
 
 EXAMPLE