fix(AIP-122): clarify shortening rules for nested collections

[noahdietz]

While implementing rules for singular/plural presence in pattern values including the nested collection exception, I noticed that the guidance was vague for each specific field in the google.api.resource annotation.

[jskeet]

Think I'd still like to hear more about the intended uses for singular and plurals in a meeting - I haven't been tracking this closely enough.

[noahdietz]

Think I'd still like to hear more about the intended uses for singular and plurals in a meeting - I haven't been tracking this closely enough.

They are used for a few different things but mostly to give an authoritative field within the annotation to define the words so that those usages of the resource singular & plural forms aren't left to guess it from the pattern or type

https://github.com/googleapis/googleapis/blob/46eb6505cc4b8340d304510d7226de96bafe9314/google/api/resource.proto#L180-L193