document names, aliases and terms

[tyrasd]

closes #28, see #28 (comment) for the reasoning behind the relatively strict rules for aliases

[westnordost]

LGTM if this is your decision. It would be very helpful to add an example that shows what is an alias and what not. Suggestions what to use as example...

for real aliases:

• harbor and (sea)port (landuse=harbor)
• bookmaker, bookie, turf accountant (shop=bookmaker)

for not-aliases:

• forestry road as only a sub-type of land-access road, not an alias (highway=track)
• barber shop as only a sub-type of hairdresser (?)

(Edit: removed not-an-alias example with daycare, preschool and kindergarten because tagging situation is difficult)