doc: Mention naming convention used by injection points

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 <[email protected]>
Reviewed-by: Aleksander Alekseev <[email protected]>
Discussion: https://postgr.es/m/OSCPR01MB14966E14C1378DEE51FB7B7C5F5B32@OSCPR01MB14966.jpnprd01.prod.outlook.com
Backpatch-through: 17