Issue title: cli/flags: Regression for fractions with units
 Issue body: The `--cache` and `--max-sql-memory` flags support several formats, including fractions, percentages, and suffixes like `GB`. #22460 broke one format that was previously working, non-integer values with a unit suffix like `1.5GiB`, because it assumes that if a dot is present, it's a fraction without units. 
 Comments: 
 Comment 0: @jseldess While we have a fix, it isn't going to be present until 2.0.1 and this bug should be documented as a known limitation.
 Comment 1: Documented as a known limitation in https://github.com/cockroachdb/docs/pull/2823.
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