apidoc: add API documentation for signing interface

Add the documentation for all public functions.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>

apidoc/ostree-docs.xml

@@ -21,6 +21,7 @@
 		<xi:include href="xml/ostree-sepolicy.xml"/>
 		<xi:include href="xml/ostree-sysroot-upgrader.xml"/>
 		<xi:include href="xml/ostree-gpg-verify-result.xml"/>
+		<xi:include href="xml/ostree-sign.xml"/>
 		<xi:include href="xml/ostree-bootconfig-parser.xml"/>
 		<xi:include href="xml/ostree-chain-input-stream.xml"/>
 		<xi:include href="xml/ostree-checksum-input-stream.xml"/>