[docs] Increase clarity around the usage of font icons (#13628)
[docs] Increase clarity around the usage of font icons (#13628)

* Fixed bug where sample code for `SendIcon`

With this current sample code, I was not able to get `SendIcon` to work.

I was able to import `SendIcon` from`'@material-ui/core/Send'` and use it to display the `SendIcon` within the button.

* [docs] Increase clarity around the usage of font icons

oliviertassinari/material-ui,mbrookes/material-ui,kybarg/material-ui,kybarg/material-ui,mbrookes/material-ui,kybarg/material-ui,mui-org/material-ui,callemall/material-ui,lgollut/material-ui,rscnt/material-ui,oliviertassinari/material-ui,Kagami/material-ui,Kagami/material-ui,rscnt/material-ui,callemall/material-ui,callemall/material-ui,rscnt/material-ui,allanalexandre/material-ui,callemall/material-ui,Kagami/material-ui,kybarg/material-ui,mui-org/material-ui,mui-org/material-ui,mbrookes/material-ui,Kagami/material-ui,oliviertassinari/material-ui