Fix more missing mentions of Set in notification docs

[mdickinson]

This PR builds on #1617 to make more extensive updates and fixes to the on_trait_change documentation:

• Many things that apply to List and Dict also apply to Set; fix missing mentions of Set
• The trait_name[] pattern applies to both List and Set (but not Dict); fix docs accordingly
• Some minor style, consistency and typographical fixes.

[mdickinson]

I'm not sure that this is right for Dict; I need to see if I can reproduce.

[mdickinson]

Nope, I can't find any way to reproduce behaviour that would be consistent with this for Dict objects.

For example, this fails to invoke the handler:

Python 3.10.3 (main, Mar 17 2022, 16:03:18) [Clang 13.0.0 (clang-1300.0.29.3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from traits.api import *
>>> class A(HasTraits):
...     foo = Dict()
... 
>>> def handler(object, name, old, new): print(object, name, old, new)
... 
>>> a = A()
>>> a.on_trait_change(handler, "foo[]")
>>> a.foo[4] = 16

[mdickinson]

... and the same is true in Traits 5.2.0, so this isn't one of the many things that we broke when introducing the new list, dict and set implementations.

[corranwebster]

LGTM modulo working out what is going on with Dicts and []