[lldb][Docs] Document our major differences from the LLVM style (#66345)

Running:
$ clang-format -i $(find -regex "\./lldb/.*\.c") $(find -regex
"\./lldb/.*\.cpp") $(find -regex "\./lldb/.*\.h")

Resulted in:
 1602 files changed, 25090 insertions(+), 25849 deletions(-)
(note: this includes tests which we wouldn't format, just using this as
an example)

The vast majority of which were whitespace changes. So as far as
formatting we're not deviating from llvm for any reason other than not
churning old code.

Formatting aside, the major features of lldb (single line if, early
return) are all reflected in llvm's style. We differ mainly on variable
naming (proposed to change in
https://llvm.org/docs/Proposals/VariableNames.html anyway) and use of
asserts. Which was already documented.