runtime(doc): cross-link :| meaning :p and explain E749

runtime(doc): cross-link :| meaning :p and explain E749

E749 is given when :print (with any range) is issued on an empty buffer,
like the one you get with :new or :enew. Furthermore, due to Vi
compatibility :| is a synonym.

As a result, mappings intended to include a <bar> separator (esp. in the
case of boolean or "||") between commands can generate E749 on startup
when placed in a vimrc if the bars are not properly encoded or escaped.
[1]. Document this failure mode and synonym near the generated error,
and cross link with :help :bar. Note that one must read or scroll quite
a bit to find the mention of :| behaving like :print!

[1]: https://vi.stackexchange.com/q/46625/10604

closes: #17173

Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>