Fix for measurement

doc/html/pcre2api.html

@@ -4048,9 +4048,18 @@ <h1>pcre2api man page</h1>
 The <b>pcre2_set_substitution_callout()</b> function can be used to specify a
 callout function for <b>pcre2_substitute()</b>. This information is passed in
 a match context. The callout function is called after each substitution has
-been processed, but it can cause the replacement not to happen. The callout
-function is not called for simulated substitutions that happen as a result of
-the PCRE2_SUBSTITUTE_OVERFLOW_LENGTH option.
+been processed, but it can cause the replacement not to happen.
+</P>
+<P>
+The callout function is not called for simulated substitutions that happen as a
+result of the PCRE2_SUBSTITUTE_OVERFLOW_LENGTH option. In this mode, when
+substitution processing exceeds the buffer space provided by the caller,
+processing continues by counting code units. The simulation is unable to
+populate the callout block, and so the simulation is pessimistic about the
+required buffer size. Whichever is larger of accepted or rejected substitution
+is reported as the required size. Therefore, the returned buffer length may be
+an overestimate (without a substitution callout, it is normally an exact
+measurement).
 </P>
 <P>
 The first argument of the callout function is a pointer to a substitute callout