doc: Mention naming convention used by injection points

All the injection points used in the tree have relied on an implied
rule: their names should be made of lower-case characters, with dashes
between the words used.

This commit adds a light mention about that in the docs, encouraging the
practice.

Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Discussion: https://postgr.es/m/OSCPR01MB14966E14C1378DEE51FB7B7C5F5B32@OSCPR01MB14966.jpnprd01.prod.outlook.com
Backpatch-through: 17

doc/src/sgml/xfunc.sgml

@@ -3615,7 +3615,8 @@ INJECTION_POINT(name);
      within the server code. After adding a new injection point the code needs
      to be compiled in order for that injection point to be available in the
      binary. Add-ins written in C-language can declare injection points in
-     their own code using the same macro.
+     their own code using the same macro. The injection point names should
+     use lower-case characters, with terms separated by dashes.
     </para>
 
     <para>