[LLDB] Clarifying the documentation for variable formatting wrt to qualifiers...

[LLDB] Clarifying the documentation for variable formatting wrt to qualifiers and adding a test that demonstrates this

When looking up user specified formatters qualifiers are removed from types before matching,
I have added a clarifying example to the document and added an example to a relevant test to demonstrate this behavior.

Differential Revision: https://reviews.llvm.org/D99827