docs: triangular dispatch

[nsajko]

This section in the manual refers to "so-called triangular dispatch": https://docs.julialang.org/en/v1.9-dev/manual/methods/#Extracting-the-type-parameter-from-a-super-type

However, the term is not explained, or used anywhere else in the docs.

From the below links I gather that the term is mainly of historical significance, and the mention should perhaps be removed from the docs to avoid unnecessary confusion. Alternatively, I guess there should be an explanation of what "triangular dispatch" means.

https://stackoverflow.com/questions/30746405/can-i-use-a-subtype-of-a-function-parameter-in-the-function-definition

https://discourse.julialang.org/t/what-is-the-triangle-in-triangular-dispatch/18817

#3766