Fix the description of the _MBC_ILLEGAL return value, especially so in the case type == _MBC_LEAD.

[WilliamCushing]

Add english descriptions of the conditions that results in illegality, and spell out the value ranges in positive fashion rather than negative fashion based on the values shown in the other rows.

Note that the table does not demonstrate locale-sensitivity or codepage-sensitivity; are the given example values the ones used for codepage 932 (japanese kanji)? Does locale really effect _mbbtype?

[prmerger-automator]

@WilliamCushing : Thanks for your contribution! The author(s) have been notified to review your proposed change.

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[ShannonLeavitt]

@TylerMSFT

Can you review the proposed changes?

IMPORTANT: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team

[TylerMSFT]

This is a really nice improvement over the inverted ranges, and the broken range

[TylerMSFT]

#sign-off
I'll add addressing your question to my backlog. It'll be a while before I can get to it.