Quicktags instances loop check for instance '0'

[afercia]

While checking for a way to count Quicktags instances, I noticed that in the instances loop the check for the instance '0' was changed in strict equality I guess to make JSHint happy, see 26212:
changed from

if ( inst == 0 ) 

to

if ( inst === 0 )

AFAIK JavaScript Object property keys (names) are always string and this always evaluates to false so in the default Edit Post screen the loop runs 3 times:
0
content
replycontent
Proposed patch checks for a string and uses "Yoda" style. Fixes a small typo in the doc.

[azaozz]

Patch looks good.

[azaozz]

In 29889:

Quicktags: fix strict check for instance['0'], props afercia, fixes #29767