Add effect to the keywords in the manual

[lupjo]

The published manual doesn't mention the effect keyword
It could also be relevant to mention the fact that the set of keywords now also depends on the -keyword option

[nojb]

Sounds reasonable. Would you like to propose a PR? Thanks!

[dra27]

We could possibly look at either mechanising the relevant section or having a check on it (a la the Standard Library modules checks in the manual), given that we also have keywords based on the syntax version.

cf. for interest, an un-merged splitting of the keywords information out of the lexer dra27@dd07f7b#diff-704f20cc226370aa24d24073df0a9d191e405b5025fe55e921101c9c581a3845

[yawaramin]

Are we sure that the manual doesn't mention the effect keyword? It was added in #13295