string: add argchecks and improve LDocs.

* specs/string_spec.yaml (assert, caps, chomp, escape_pattern)
(escape_shell, finds, format, ltrim, monkey_patch, numbertosi)
(ordinal_suffix, pad, prettytostring, render, require_version)
(rtrim, split, tfind, trim, wrap): Specify behaviours with bad
or missing arguments.
* lib/std/string.lua (assert, caps, chomp, escape_pattern)
(escape_shell, finds, format, ltrim, monkey_patch, numbertosi)
(ordinal_suffix, pad, prettytostring, render, require_version)
(rtrim, split, tfind, trim, wrap): Use argcheck or argscheck to
ensure argument types are validated when _DEBUG is not `false`.
Improve LDocs with @usage examples and parameter types.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>