The tooltips added for shader nodes in D15309 are very helpful already.
In this patch I propose some tweaks to make them more consistent with
the general purpose of tooltips, more consice, and more accurate. There
are a few fixes included as well.

Here is a summary of the changes:

• Use more consise wording using fewer unnecessary words
  • Start fewer new sentences
  • Use "For Example" slightly less
  • Avoid repeating the node's name unnecessarily
• Don't capitalize some words
• Spelling/grammar fixes
  • Use consistent verb conjugation
  • Use ASCII characters/more common quote symbols
• Refer to color sockets as colors rather than "images"
• Corrections to information
  • Plural/singular corrections, i.e. "vector curve*s*"
  • "smoke domains" -> "volume grids"
  • Fix tooltip for separate and combine color nodes
• Avoid "advice" in a few places, which should be left for the manual
• Remove information for specific sockets and could be in their tooltips
• Avoid refering to the specific location of a property in the UI
• Avoid specifying newlines in some cases (mostly reserve for "Note:")
  • This leaves the newlines more in control of the UI code and is generally more consistent with tooltips elsewhere.
• Add some information
  • That the UV map and color attribute nodes use a default
  • That Voronoi is "based on the distance to random points"
  • Add "(Deprecated)" to old color combine and separate nodes

I do understand that this might take some of the descriptions further away
from the wording in the manual. But I do think it's worth it to change these
a bit more, since there are different standards and practices for UI tooltips.