feat(docs): add deprecation notice to methods and constants

[bshaffer]

fixes #6792

adds deprecated notice to classes, methods, constants, parameters, and nested parameters

• Classes can now receive this, but we exclude deprecated classes from the reference documentation. This was done because when we first published the reference documentation, there was no need for deprecated classes. Once we release a GAX and Auth library 2.0, it would make sense to add them back. Once we do, the deprecation notices will show up.
• Parameters and Nested Parameters can now receive this, but it's important to note there's no consistent way in phpdoc to mark a parameter as deprecated. The closest we have is to prefix the description with [DEPRECATED]. In protobuf, sometimes this is after the keyword Optional, so if [DEPRECATED] shows up anywhere in the parameter or nested parameter description, it's considered deprecated.
• Constants and methods look for @deprecated as expected, but this should be at the END of the constant or method description, otherwise the notice_content will contain the phpdoc content instead of just the deprecation content.