Changing Value of `gnus-use-cache'

Out of ignorance, I have had `gnus-use-cache' set to `t' for a *long* time.

Rereading the documentation (info), indicates that a value of
`passive' would probably be better.  That value will only cache
articles that are "explicitly entered".

Some questions:

   If I change it to `passive', will the unnecessarily cached articles
   be automatically removed?  Or, must I do that myself.

   What does "explicitly entered" mean? ... only via the `!' and `?'
   commands?  What about `*'?

   Should I "tweak" the values of `gnus-cache-enter-articles' and/or
   `gnus-cache-remove-articles'?  Also, 'gnus-cacheable-groups'?

   Anything else I should watch out for if I make this change?

Reading the above leads me to realize that I probably should state
what I *want* to happen. 

Basically, I want to avoid any unnecessary caching (I'm using `nnml'
"under my home directory"), and be able to retain ("forever")
newsgroup articles (others?) that are of interest.

Writing the above (and referring back to the documentation) has given
me some good ideas of how to change my configuration ... however, I'm
still interested in hearing from others.