What should textplot_network() do with asymmetric fcm?

[koheiw]

In relation to quanteda/quanteda#1461, we haven't defined how to deal with an asymmetric fcm. Probably, the lower triangle is ignored in the conversion:
https://github.com/quanteda/quanteda/blob/fe6af954ad31ea9e47b8baacc95a9dd9c8468ca7/R/textplot_network.R#L153-L154
I should warn users or raise error when a fcm is asymmetric.

[kbenoit]

This will probably not arise often, because few users will set ordered = TRUE if they are looking to produce network plots. However if they do, I think a warning is appropriate.