rust: bitmap: document panics in `next_bit` and `next_zero_bit`

`next_bit()` and `next_zero_bit()` use `bitmap_assert!()` to check
that `start` is less than `self.len()`, which panics when
`CONFIG_RUST_BITMAP_HARDENED` is enabled. However, neither function
has a `# Panics` section, and both document that `None` is returned
for exactly the input that triggers the panic.

Update the documentation of `next_bit()` and `next_zero_bit()` to add
the missing `# Panics` sections, add the missing blank doc comment line
before `Returns` in `next_zero_bit()`, and simplify the wording.

Fixes: 11eca92a2cae ("rust: add bitmap API.")
Closes: https://github.com/Rust-for-Linux/linux/issues/1252
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Georgios Androutsopoulos <georgeandrout13@gmail.com>
Signed-off-by: Yury Norov <ynorov@nvidia.com>

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