minor doc change in src/sgml/xfunc.sgml

hi.
simlar to https://www.postgresql.org/docs/devel//xfunc-c.html#XFUNC-ADDIN-WAIT-EVENTS


in https://www.postgresql.org/docs/devel//xfunc-c.html#XFUNC-ADDIN-INJECTION-POINTS
do we need to add a sentence like " An injections points usage example
can be found in src/test/modules/injection_points in the PostgreSQL
source tree."
before "Enabling injections points requires --enable-injection-points
with configure or -Dinjection_points=true with Meson."
?