[MLIR][Python] add/fix docstrings in IRCore (llvm#167063)

This PR adds all the missing doc strings in IRCore.cpp. It also

1. Normalizes all doc strings to have proper punctuation;
2. Inlines non-duplicated docstrings which are currently at the top of
the source file (and thereby possibly out of sync).

Follow-up PRs will do the same for the rest of the modules/source files.

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Co-authored-by: Copilot <[email protected]>

Author: makslevental