Fix invalid range validators.

[anntzer]

• The correct bound on margins value, documented in set_x/y/zmargin, is margin>-0.5. Fix the docstring of margins() and the rc validators.
• The only constraints on tight/constrained layout margins is left<right and bottom<top, but values outside of [0, 1] can in fact be valid. It would be annoying to enforce this at the rcparams level (this would force the user to update individual rcparams in a careful order), so just don't bother validating the values beyond "float". (See discussion at Group figure.subplot.* rc to a single rcParam. #11231 (comment))
• Remove the somewhat overengineered _range_validators.
• Move the comment about the values of figure.constrained_layout.h/w_pad to the default matplotlibrc (in rcsetup.py it was far away from the actual values, which made things a bit weird). Also reword a bit.

PR summary

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines