doc string-pad string-pad-right

[Kevin Ryde]

Where the doc for string-pad says it's "right-padded" it makes me
think it does its padding on the right end, but in fact it's on the
left.  Maybe it was trying to say something like "right-justified" in
the field.  In any case I gave it and string-pad-right a bit of
polish.

The possible sharing described is per srfi-13.  This had been saying
the return was a "new string", but I'd like to quietly drop that in
favour of what the srfi says.


 -- Scheme Procedure: string-pad s len [chr [start [end]]]
 -- Scheme Procedure: string-pad-right s len [chr [start [end]]]
 -- C Function: scm_string_pad (s, len, chr, start, end)
 -- C Function: scm_string_pad_right (s, len, chr, start, end)
     Take characters START to END from the string S and either pad with
     CHAR or truncate them to give LEN characters.

     `string-pad' pads or truncates on the left, so for example

          (string-pad "x" 3)     => "  x"
          (string-pad "abcde" 3) => "cde"

     `string-pad-right' pads or truncates on the right, so for example

          (string-pad-right "x" 3)     => "x  "
          (string-pad-right "abcde" 3) => "abc"

     The return string may share storage with S, or it can be S itself
     (if START to END is the whole string and it's already LEN
     characters).