docs: minor documentation fixes

[AaronFeickert]

This PR makes a few small documentation fixes:

• It updates the ristretto module documentation to correctly note that RistrettoPoint equality testing is always done in constant time.
• It updates the edwards module documentation to correctly note that EdwardsPoint equality testing is always done in constant time.
• It updates the EdwardsPoint documentation to fix what appears to be a copy-paste error for a type name.

[rozbb]

Indeed you're right! Thank you for the fixes