pallet minor doc improvment (#7922)

* doc improvment

* additional fixes

* another fix

* better code suggestion

* Apply suggestions from code review

Co-authored-by: David <dvdplm@gmail.com>

* Apply suggestions from code review

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Apply suggestions from code review

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* apply suggestion

* apply suggestion

* Update frame/support/src/lib.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* apply suggestion

* better guideline on reexport

* Update frame/support/src/lib.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Update frame/support/src/lib.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Update frame/support/src/lib.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Update frame/support/src/lib.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Update frame/support/src/lib.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* apopiak suggestion

* clearer check suggestion

* Update frame/support/src/lib.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Update frame/support/src/lib.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Update frame/support/src/lib.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

Co-authored-by: David <dvdplm@gmail.com>
Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>