Flesh out the documentation for Mercator

This addresses item 1 in issue #2387

Things to note:

* I made "editorial" changes to the text.  The virtues and vices of
  Mercator are a hot topic.  So check these out.  (I judged that the
  text I replaced to be pretty misleading.)

* I include the radius of the sphere/ellipsoid in the formulas (and I
  did this also for my mods for tmerc documentation).  Surely this is
  better than leaving the reader to figure out how this is introduced.

* I include the "old-style" (ca 18th century) formulas and the newer
  ones in terms of hyperbolic functions.  The former may be the familiar
  ones, but the latter are better for computation (more succinct, more
  accurate, faster, preserve parity).

* For the inverse ellipsoidal transformation, I just say that the
  formula for psi is inverted iteratively.  This is probably sufficient,
  but it could be expanded later.

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