Improve abilities documentation

[soliton-]

https://wiki.wesnoth.org/AbilitiesWML is severely lacking and needs to be improved.

• describe dummy abilities (those that are not defined by the engine)
  • describe how dummy abilities can be used together with events to implement actual gameplay effects with example(s)
• describe how abilities that use a value key work.
• describe all keys/tags that are related to how value works
• describe all keys/tags that are common to all abilities including dummy ones
• describe all keys/tags that in addition to the above are common to all engine-defined abilities
• improve description on how weapon specials as abilities work
• improve description of how cumulative works
• link and improve [filter_ability] documentation
• document add/sub/multiply/divide for abilities that do actually support it

[newfrenchy83]

https://wiki.wesnoth.org/AbilitiesWML is severely lacking and needs to be improved.

* [ ]  describe dummy abilities (those that are not defined by the engine)
  
  * [ ]  describe how dummy abilities can be used together with events to implement actual gameplay effects with example(s)

* [ ]  describe how abilities that use a `value` key work.

* [ ]  describe all keys/tags that are related to how `value` works

* [ ]  describe all keys/tags that are common to all abilities including dummy ones

* [ ]  describe all keys/tags that in addition to the above are common to all engine-defined abilities

* [ ]  improve description on how weapon specials as abilities work

* [ ]  improve description of how cumulative works

* [* ]  link and improve [filter_ability] documentation

i added https://wiki.wesnoth.org/StandardAbilityFilter page for experimental_filter_ability

[KilluaZoldyck133]

How to send in your suggestions for changes to the wiki, is there an email address or some site?

[soliton-]

You could write them here or edit the wiki and post a link to the edit.

[Pentarctagon]

Added another item since with #8074 I discovered that add/sub/multiply/divide are in fact used by heals. Presumably those are actually common keys, at least for abilities that have a value.