Format of Effort estimates should be mentioned in its Info node

[Kisaragi Hiu]

Currently, the Info node about effort estimates does not mention what
format should it be written in. This causes confusion, as a user might
assume that it's the same format as schedulers (10m, 6h, etc.) like I
did.

Simply mentioning "Effort estimates need to have the format H:MM"
(copied from the docstring or org-effort-property, the only*
description of the format I could find) in the Info node would be
enough.


*: searching for regexp effort.*format and manually filtering out the other 3 
matches