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>

tools/binman/btool/pkcs11_tool.py

@@ -7,7 +7,16 @@ from binman import bintool
 
 
 class Bintoolpkcs11_tool(bintool.Bintool):
-    """pkcs11-tool -- support tool for managing pkcs#11 tokens"""
+    """pkcs11-tool -- support tool for managing pkcs#11 tokens
+
+    This bintool wraps the `pkcs11-tool` command from the OpenSC project for
+    managing objects stored in PKCS#11 tokens. Binman uses this wrapper only
+    to check that pkcs11-tool is installed (and to fetch it if missing); any
+    actual key or token management for signing FIT images or capsules is done
+    outside binman.
+
+    See https://github.com/OpenSC/OpenSC/wiki for more details.
+    """
     def __init__(self, name):
         super().__init__('pkcs11-tool',
                          'PKCS #11 tokens managing tool',