core: replace `:additional-docs` by an independent macro
core: replace `:additional-docs` by an independent macro

- New macro `spacemacs|transient-state-format-hint`
Additional hints can be formatted outside of the transient state macro.
This is a better separation of concern from an API point of view
because those hint are coupled to with the usage of :dynamic-hint
keyword. The transient state macro only need to know the entry point
for such dynamic hints, effectively increasing the readability of
transient state definition.

To be able to decouple the formatting of additional hints, a new
private variable whose name is given by the function
`spacemacs//transient-state-props-var-name` has been introduced.
An alternative would have been the save the whole transient state macro
props but a variable allows the cherry-pick only the values we really
need.

- Remove commented code about old variable
`spacemacs-transient-state-title-face`

- Remove unused keyword :entry-binding since it is not used. Given
the number and diversity of transient states in Spacemacs currently it
is a safe guess to remove this keyword for now. We can reintroduce it
later if really needed.

tsonntag/dotemacs,tsonntag/dotemacs,tsonntag/dotemacs,tsonntag/dotemacs