dox feature can break request_pad_simple method call syntax

There are two request_pad_simple methods, one on ElementExt and one on ElementExtManual. They're supposed to be mutually-exclusive AFAICT, but when using the dox feature they are not. I think an intended use case for that feature is being able to run checks against one's code w/o having gstreamer installed, which is what we're doing. If that's correct this should probably be fixed, likely using the cfg(docsrs) trick¹ to still have both definitions show up on docs.rs.

¹ this:

[package.metadata.docs.rs]
rustdoc-args = ["--cfg", "docsrs"]