docs: improve description/naming for flat graph merge_in

[MingweiSamuel]

The description I wrote of merge_in is not very helpful, I think we should point out that the Ends it returns are the input and output of the module that has been imported into self.

I also think that merge_in is kind of a misleading name, it's not really doing any 'merging'. I think a better name might be 'import_flatgraph', since what it returns is a handle to the imported flatgraph, and that handle can be used to complete the stitching.

Not sure about the second idea, but the first idea I think would be helpful.

Originally posted by @zzlk in #1017 (comment)

[MingweiSamuel]

#1110