In systems that use the MAWK interpreter (the default AWK installed with
Ubuntu), the output of `rbenv help <command>` would have no line breaks.
The issue is fixed by changing `gsub` to `sub` in the snippet of awk
commands that are used to extract documentation comments.
I suspect the bug is something to do with the way the '^' and '$'
characters are interpreted by different AWK interpreters (per-line vs
per-string anchors).
If I understand correctly, the purpose of trim() is to remove all line
breaks from the start and end of each sections of a command's
documentation, in which case `sub` should serve the same purpose.
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