When adding a modifier to an object, the drop-down select box has the same mouse-over tooltip for each modifier type: "Add a procedural operation/effect to the active object". This isn't helpful when you don't know what the modifier does from just reading the name.
This patch adds descriptions for most modifier types, so that it's clearer what the modifiers do. The text was provided by @Julien Blervaque (werwack) after discussion on DevTalk and taken from the Blender Manual. This process could in a sense be automated and either extract the manual text from the source code or vice versa, but I think it's a better idea to keep both separate. That way tooltips can be altered to make them more succint, whereas the manual text can utilise the larger space available there.
New tooltips:
Not all modifiers have a short one-line description in the manual, hence those are still missing in this patch.
Note that, contrary to operator tooltips on buttons and menus, the trailing period is not automatically added by Blender. Do we want to add those to the description explicitly (easier to do now)? Or do we want Blender to add those automatically (to be consistent).