verify: small refactoring and documentation in `_verifymanifest` Small changes to make this area of code clearer.

verify: document the `_verifymanifest` method

verify: document `_verifychangelog` We document the method input, output and checks.

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.

verify: make `checkentry` a private method This method is for internal use only.

verify: document the `checkentry` method This method checks various core propertes of a revision. We document inputs, outputs and the checks performed.

verify: add some inline documentation to the top level `verify` method The goal is to clarify each section goal.

verify: explicitly return 0 if no error are encountered Relying on the fact None is treated as 0 by other logics seems smarter than we should be.