DOC: Clarify that method reconstruction methods "parsimony"="fitch" and "probabilistic"="ml"

[corneliusroemer]

Going through the source code I realised that 2 each of the 4 possible ancestral reconstruction methods are synonyms.

Note to future self before I forget: This could be explained explicitly in the documentation.

Mapping is:

"parsimony"="fitch"
"probabilistic"="ml"