in 9.7.2 do you think it's worthwhile changing it to
"" As with SIMILAR TO, substring(string, pattern, escape-character) the specified pattern must match the entire data string, or else the function fails and returns null. "" ?
Making reference to any one of the three listed function signatures here doesn't seem to provide value. If anything I'd write:
"As with SIMILAR TO, substring matches the specified pattern to the entire data string, returning null otherwise."
I would avoid saying that the function fails in any situation that doesn't produce an actual error. The transition of "match everything or returns null" can be bike-shedded though.