nls: harmonize the comments for compose sequences with combining accents

Benno Schulenberg requested to merge benno/libx11:lowercasewith into master

Put the "with" and the "and" in comments for sequences with combining accents into lowercase, to make it slightly clearer that the resulting string consists of multiple code points. Also, use the word "COMBINING" before the name of the first accent, so that these sequences can be easily grepped, and drop the redundant word "ACCENT".

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