Make lightning::sync public

[AnthonyRonning]

I believe it might have always been the intention. I tested this 116-alpha1 fork out on Mutiny and sending / receiving seems to be working, just needed pub here.

Needed so that I could use some of the no_std mutex stuff in the sync package. Judging by all the pub structs and functions in sync, maybe it was just an oversight that it wasn't public before? If it needs to be private, I may need some help figuring out how to work around it.

Also, I think a handful of things need docs. I really don't know what to say about them. Happy to put whatever you guys recommend or let maintainers edit this PR directly.

error: missing documentation for a module
   --> lightning/src/lib.rs:182:1
    |
182 | pub mod sync;
    | ^^^^^^^^^^^^
    |
note: the lint level is defined here
   --> lightning/src/lib.rs:40:68
    |
40  | #![cfg_attr(not(any(test, fuzzing, feature = "_test_utils")), deny(missing_docs))]
    |                                                                    ^^^^^^^^^^^^

error: missing documentation for an associated function
  --> lightning/src/sync/fairrwlock.rs:22:2
   |
22 |     pub fn new(t: T) -> Self {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^

error: missing documentation for a method
  --> lightning/src/sync/fairrwlock.rs:29:2
   |
29 |     pub fn write(&self) -> LockResult<RwLockWriteGuard<T>> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: missing documentation for a method
  --> lightning/src/sync/fairrwlock.rs:36:2
   |
36 |     pub fn read(&self) -> LockResult<RwLockReadGuard<T>> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: missing documentation for a method
  --> lightning/src/sync/fairrwlock.rs:49:2
   |
49 |     pub fn try_write(&self) -> TryLockResult<RwLockWriteGuard<'_, T>> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[wpaulino]

Closing in favor of #2383.