Documentation/learning/api.md: fix markup, wording
Documentation/learning/api.md: fix markup, wording

A few words have been emphasized to be consistent with the rest
of the text. Also, some phrases have been altered for better
readability.

cockroachdb/etcd,etcd-io/etcd,whuwxl/etcd,sebrandon1/etcd,vimalk78/etcd,mkumatag/etcd,heyitsanthony/etcd,sebrandon1/etcd,whuwxl/etcd,li-ang/etcd,xiang90/etcd,mkumatag/etcd,mqliang/etcd,bdarnell/etcd,sinsharat/etcd,mkumatag/etcd,coreos/etcd,ajityagaty/etcd,rnaveiras/etcd,fanminshi/etcd,rnaveiras/etcd,li-ang/etcd,bluebreezecf/etcd,dannysauer/etcd,lorneli/etcd,rnaveiras/etcd,mqliang/etcd,fanminshi/etcd,vimalk78/etcd,bluebreezecf/etcd,dannysauer/etcd,gyuho/etcd,ajityagaty/etcd,lorneli/etcd,cockroachdb/etcd,bdarnell/etcd,ajityagaty/etcd,coreos/etcd,glevand/coreos--etcd,mqliang/etcd,xiang90/etcd,sinsharat/etcd,etcd-io/etcd,joshix/etcd,joshix/etcd,heyitsanthony/etcd,gyuho/etcd,etcd-io/etcd,glevand/coreos--etcd