This is a very small PR that adds some lock annotations to clarify that, now, the pool.cs lock is held throughout tx validation for mempool. The comments in CheckInputsFromMempoolAndCache were unclear/outdated so I updated those as well.

This is a backport of core#20834

Notes: the PolicyScriptChecks and CheckFeeRate methods don't exist in our codebase, they are part of PreChecks. See D8203.

With clang and debug:
ninja all check-all

With clang and TSAN:
ninja check check-functional