Correct 'Indexing extra fields' note to avoid implying that the Postgresql backend doesn't support it
Correct 'Indexing extra fields' note to avoid implying that the Postgresql backend doesn't support it

Fixes #3899 - thanks @sivictorallen for the report!

timorieber/wagtail,rsalmaso/wagtail,rsalmaso/wagtail,mixxorz/wagtail,mikedingjan/wagtail,jnns/wagtail,nimasmi/wagtail,wagtail/wagtail,nealtodd/wagtail,torchbox/wagtail,thenewguy/wagtail,jnns/wagtail,gasman/wagtail,wagtail/wagtail,torchbox/wagtail,nealtodd/wagtail,takeflight/wagtail,thenewguy/wagtail,rsalmaso/wagtail,timorieber/wagtail,mixxorz/wagtail,takeflight/wagtail,kaedroho/wagtail,mikedingjan/wagtail,torchbox/wagtail,mixxorz/wagtail,FlipperPA/wagtail,kaedroho/wagtail,jnns/wagtail,zerolab/wagtail,mikedingjan/wagtail,gasman/wagtail,gasman/wagtail,kaedroho/wagtail,zerolab/wagtail,nimasmi/wagtail,nealtodd/wagtail,rsalmaso/wagtail,FlipperPA/wagtail,takeflight/wagtail,nealtodd/wagtail,FlipperPA/wagtail,FlipperPA/wagtail,mixxorz/wagtail,zerolab/wagtail,mixxorz/wagtail,thenewguy/wagtail,rsalmaso/wagtail,thenewguy/wagtail,gasman/wagtail,mikedingjan/wagtail,wagtail/wagtail,gasman/wagtail,timorieber/wagtail,nimasmi/wagtail,wagtail/wagtail,takeflight/wagtail,kaedroho/wagtail,zerolab/wagtail,kaedroho/wagtail,timorieber/wagtail,wagtail/wagtail,thenewguy/wagtail,torchbox/wagtail,jnns/wagtail,nimasmi/wagtail,zerolab/wagtail