[latexrefman] Add section on \caption

[Karl Berry]

Sorry to be chipping in, but for myself, I have trouble with
"blank chararacter". It just doesn't sound right. I think
"whitespace character" is much more common, when needing to speak
technically of any whitespace char.

However, what I think would actually make the most sense is to say at
the beginning that "spaces" generally means "any whitespace character",
and in the (very few) cases where space characters must be distinguished
from newlines distinguished from ..., we say so explicitly. That would
result in much less verbosity -> better readability, it seems to me. -k