Document tiered percentage in doc/api.md (#1307)
Document tiered percentage in doc/api.md (#1307)

* Document tiered percentage in doc/api.md

Simplify finding information about the `tiered_percentage`
field by specifically discussing it in doc/api.md.

Signed-off-by: David A. Wheeler <9ae72d22d8b894b865a7a496af4fab6320e6abb2@dwheeler.com>

* Document in doc/api.md when to use badge_level

Mention in doc/api.md that
if you just want the text of the earned badge, use `badge_level`.

Signed-off-by: David A. Wheeler <9ae72d22d8b894b865a7a496af4fab6320e6abb2@dwheeler.com>

coreinfrastructure/best-practices-badge,coreinfrastructure/best-practices-badge,linuxfoundation/cii-best-practices-badge,linuxfoundation/cii-best-practices-badge,linuxfoundation/cii-best-practices-badge,coreinfrastructure/best-practices-badge,linuxfoundation/cii-best-practices-badge,coreinfrastructure/best-practices-badge