How do I read a species description that uses both ≡ (triple bar) and "incl." to denote synonymy?
According to the Taxon guidelines for authors (pdf, pg 205), a triple-bar (≡) indicates a homotypic synonym:


  Homotypic names are cited in chronological order in a single paragraph with 
  the identity sign (≡), followed by the type.


So, in your example, Dorstenia foetida was described on the basis of the same type specimen used by Forsskål in 1775 to describe Kosaria foetida -- or possibly a neotype was designated linking these two names, possibly by Schweinfurth in Bull. Herb. Boissier 4(App. 2): 120 in 1896. More information about the relevant type specimens would help sort this out.