doc: Refine the command line parsing doc for C (#1010)

Improve punctuation. Fix some grammar. Put which/that right. Reword some
sentences for clarity. Fix a few broken references. Lose some passive
voice. Relabel two sections to match the text. Wrap all structure member
names with <b>, wrap all inline C code fragments with <tt>. Amend the
example C code to get the whitespace around "set"/"not set" right in all
cases. Consistently use $ as the command line prompt in all CLI examples.
Use <th> for table header rows. Show answers[] elements as strings, not
integers.