binman: Flesh out the pkcs11-tool bintool docstring

binman: Flesh out the pkcs11-tool bintool docstring

The Sphinx-generated bintools.rst currently produces an empty section
for this bintool, since its class docstring is only a single line and
so the body under the heading is blank.

Extend the docstring with a short description of what pkcs11-tool does
and how binman uses it, so the generated documentation has useful
content.

Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>