fix manual for .nosideeffect

[timotheecour]

• closes func doesn't check for side effects in proc parameters #16303
  the manual is now consistent with the implementation; the implementation was already doing the correct thing, as mentioned in func doesn't check for side effects in proc parameters #16303 (comment)
• add regression test

this enables for eg procs like sort to be func, yet still allow passing in callbacks that may have side effects; if the callback does have side effects, the resulting proc would then not be usable inside a nosideEffect context.

future work

• clarify effects through proc params for other annotations besides .nosideeffect