Add a little additional documentation on how to emulate pcre2_substitute's loop (#735)

We won't implement more advanced/alternative global replacement strategies, but we can at least write a few sentences explaining how to do it in application code.

Author: NWilson

doc/html/pcre2api.html

@@ -3742,7 +3742,8 @@ <h2><a name="SEC37" href="#TOC1">CREATING A NEW STRING WITH SUBSTITUTIONS</a></h
 end before it starts are not supported, and give rise to an error return. For
 global replacements, matches in which \K in a lookbehind causes the match to
 start earlier than the point that was reached in the previous iteration are
-also not supported.
+also not supported. (These cases are only possible if the pattern was compiled
+with the backwards-compatibility option PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK.)
 </p>
 <p>
 The first seven arguments of <b>pcre2_substitute()</b> are the same as for
@@ -3875,6 +3876,18 @@ <h2><a name="SEC37" href="#TOC1">CREATING A NEW STRING WITH SUBSTITUTIONS</a></h
 set in the match context, searching stops when that limit is reached.
 </p>
 <p>
+Because global substitutions apply the pattern repeatedly to the subject string,
+and always iterate over non-overlapping matches, the substitutions done by
+<b>pcre2_substitute()</b> do not match and substitute text inside the replacement
+strings themselves (no recursive/iterative substitution). However, applications
+can easily implement other alternative replacement strategies, such as
+iteratively replacing, then matching and replacing on the result. The
+replacement loop inside <b>pcre2_substitute()</b> is simple and can be emulated
+in client code by allocating a buffer, searching for matches in a loop, and
+calling <b>pcre2_substitute()</b> with PCRE2_SUBSTITUTE_REPLACEMENT_ONLY an
+PCRE2_SUBSTITUTE_MATCHED, and without PCRE2_SUBSTITUTE_GLOBAL.
+</p>
+<p>
 You can restrict the effect of a global substitution to a portion of the
 subject string by setting either or both of <i>startoffset</i> and an offset
 limit. Here is a <b>pcre2test</b> example: