Hi.

Here are some minor comments in reading order:

The "Table of contents" and the section headings are not aligned
(e.g. "Procedure index" vs. "Procedure Index").

Headings should be cased consistently, for example
"Optional _P_rocedures"
"References and _L_links"
should be changed (or the others should be adjusted the other way around).

string-trim-both:
"on the left / on the right / on both sides" ->
"on the left side / on the right side / on both sides"

string-replace:
"(string-set "String-ref ... 19 ...)" ->
"(string-set "String-ref ... 21 ...)"

string-append:
Why is this not marked with "(R5RS)"?

string-concatenate-reverse:
"The optional end argument accomodates that use case."
I don't understand this from the sketched Rationale.
Why can't the caller use (string-take final-string end)
as the only optional argument so that we don't need end
in the specification?

string-split:
the paragraph break before "This will often" might irritate some readers.

write-string: ... -> g
What is g?

string-set!, string-fill!, string-copy! are all marked as
"-> string"
AND
"Returns an unspecified value."
I think that only the latter is correct.

string-copy!: "atat"
Please add a space.

Greetings
Sven