Improve AltAz documentation
Improve AltAz documentation

Mention that the local horizon is defined with respect to the WGS84
reference ellipsoid, since that is what ERFA does underneath (see
astropy.coordinates.erfa_astrom for more details). This is useful to
know because other libraries might make different choices. For example,
GSFC Calc uses the IERS 2003 ellipsoid.

Mention that altitude == elevation angle, AltAz == AzEl and zenith
angle == zenith distance, as those terms are in common use in astronomy.

Also improve grammar and fix a typo.

mhvk/astropy,aleksandr-bakanov/astropy,larrybradley/astropy,mhvk/astropy,aleksandr-bakanov/astropy,saimn/astropy,mhvk/astropy,dhomeier/astropy,saimn/astropy,mhvk/astropy,saimn/astropy,saimn/astropy,larrybradley/astropy,astropy/astropy,larrybradley/astropy,StuartLittlefair/astropy,astropy/astropy,astropy/astropy,aleksandr-bakanov/astropy,pllim/astropy,StuartLittlefair/astropy,lpsinger/astropy,dhomeier/astropy,dhomeier/astropy,lpsinger/astropy,pllim/astropy,lpsinger/astropy,pllim/astropy,larrybradley/astropy,pllim/astropy,StuartLittlefair/astropy,dhomeier/astropy,aleksandr-bakanov/astropy,pllim/astropy,larrybradley/astropy,astropy/astropy,StuartLittlefair/astropy,saimn/astropy,mhvk/astropy,dhomeier/astropy,lpsinger/astropy,astropy/astropy,lpsinger/astropy,StuartLittlefair/astropy