verify: rename the `checklog` to `_checkrevlog` The method is for internal use only. In addition we make the method name explicitly contains `revlog` to make it clearer it is checking higher level revlog properties.

verify: document the `checklog` method Let us add details about what the function is expected to do.

revlog: add some documentation to the `checksize` method I had to look at it, so I figured I would leave some documentation for the next person seeking information.