fix type err, improve readability of patmat doc

[bjornregnell]

based on discussion here: #16131

[julienrf]

Does that overlap with #15993?

[som-snytt]

Indeed. The description here is not accurate about A, but the other PR also does not explain about the type param.

[Sporarum]

I will update the other PR with more information about the types

Can this PR therefore be closed ?
(But not merged or deleted, I'll use it as inspiration)

[bjornregnell]

ok I'll close this. @som-snytt for curiosity: what would be a more accurate statement about A?
Not that the other PR did not fix the unknown T (should be A) in the unapply signature.