Tweak wording in manual
Tweak wording in manual

Matt Leotta pointed out that the correct technical term (which eluded me
at time of original writing) for the type of transformation applied by
"alignment" (i.e. maptk::canonical_transform) is a "similarity"
transformation.

Also, add some text to better imply what underlying C++ API is used for
this tool.

linus-sherrill/maptk,linus-sherrill/maptk,linus-sherrill/maptk,linus-sherrill/maptk,linus-sherrill/maptk