Update blending and masking documentation

[hlecleme]

Small update of the blending and the masking options to document the changes of https://github.com/darktable-org/darktable/pull/8217

• document the "reverse" blending
• remove the specific reverse blending modes
• add "blend fulcrum" to the color channel modes
• update the feathering guide choices

[elstoc]

Looks good to me. Thanks @hlecleme!