Add small doc-comment to entry_point! macro

[georglauterbach]

Without this change, clippy will complain

error: missing documentation for a function
  --> src/main.rs:60:1
   |
60 | bootloader::entry_point!(kernel_start);
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: the lint level is defined here
  --> src/main.rs:27:9
   |
27 | #![deny(missing_docs)]
   |         ^^^^^^^^^^^^
   = note: this error originates in the macro `bootloader::entry_point` (in Nightly builds, run with -Z macro-backtrace for more info)

error: could not compile `kernel` due to previous error

when using #![deny(clippy::cargo)] and I cannot compile my kernel.

[phil-opp]

Thanks!