Font IDs guide improvements

[sylbru]

The step-by-step guide for finding font IDs has two issues that make it, I think, less clear that it should be:

• "Step 4 : Locate the Font ID" doesn’t say which specific kind of element to look for, whereas it should say (I think) that we’re looking for a style element with an id like custom-… (containing @font-faces etc.) (the ID shape is mentioned in the next step but feels at least as relevant in this one)
• In my case the UI was asking for three IDs, three variants of the same font. I could only find one font ID (one style block) for all variants of the font, so I used the same ID in all three inputs. Not sure if that’s expected as the guide didn’t mention that.
• In my case, there are a few more variants, and one more font, than those listed in the dialog that’s asking for font IDs, so it’s a bit confusing, and nothing seems to clear that up.

Sorry if that’s a bit of vague/composite issue, let me know if you want me to split that in several issues.