basic/extract-word,man: clarify "correction" of invalid escapes

Our warning message was misleading, because we wouldn't "correct" anything,
we'd just ignore unkown escapes. Update the message.

Also, print just the extracted word (which contains the offending sequences) in
the message, instead of the whole line.

Fixes #4697.

src/basic/extract-word.c

@@ -227,8 +227,8 @@ int extract_first_word_and_warn(
                 *p = save;
                 r = extract_first_word(p, ret, separators, flags|EXTRACT_CUNESCAPE_RELAX);
                 if (r >= 0) {
-                        /* It worked this time, hence it must have been an invalid escape sequence we could correct. */
-                        log_syntax(unit, LOG_WARNING, filename, line, EINVAL, "Invalid escape sequences in line, correcting: \"%s\"", rvalue);
+                        /* It worked this time, hence it must have been an invalid escape sequence. */
+                        log_syntax(unit, LOG_WARNING, filename, line, EINVAL, "Ignoring unknown escape sequences: \"%s\"", *ret);
                         return r;
                 }