Add custom taxonomy note in the tags endpoint reference

[ChrisHardie]

Right now it's difficult to find information about working with terms in a custom taxonomy via the REST API. There may be a better place for this, but my hope is that by adding a note to the tags page where developers may naturally look for that information, we can get them to the right documentation faster.

[kadamwhite]

This is an extremely good idea, but we're a bit hampered by the fact that the /reference directory is auto-generated from the core code, so if we commit this it will be removed the next time we refresh the generated documentation. Will leave this open while we review the generation system to figure out if we can make a space for this type of content.