OK, so there’s a problem with Codes in this part, I guess.
The Codex article on Next and Previous Links contradicts the article
specifically on next_post_link()
In that article you can clearly see, that it looked a little bit different. In the part describing next_post_link there is a note:
Deprecated: previous_post() and next_post(). Use: –>
previous_post_link() and next_post_link() instead.
So most probably it is describing some old params…
On the other hand, from PHP point of view…
To specify a boolean literal, use the keywords TRUE or FALSE. Both are
case-insensitive.
So true === TRUE and false === FALSE.
On the PSR-2 standard requires true, false and null to be in lower case.