Metadata V15: Expose pallet documentation (#13452)

* frame/proc: Helpers to parse pallet documentation attributes

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* frame/proc: Expand pallet with runtime metadata documentation

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* frame/dispatch: Implement doc function getter for dispatch

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* frame/tests: Check exposed runtime metadata documentation

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* frame/tests: Add UI tests for `pallet_doc` attribute

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* frame/proc: Document pallet_doc attribute

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* frame/support: Use `derive_syn_parse`

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Update frame/support/procedural/src/lib.rs

Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>

* frame/support: Improve documentation

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: parity-processbot <>
Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>