docs(icon): add note about HttpClientModule requirement

[crisbeto]

Adds a note to the icon docs that the HttpClientModule needs to be included. Also does some minor cleanup in the overview doc.

[jelbourn]

I couldn't help myself to changing a bit more; here's my proposed edits jelbourn@c19cea4

[crisbeto]

Done.

[jelbourn]

LGTM

[ngbot]

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