[Tagging] New description of waterway=pressurised

[Joseph Eisenberg]

I've updated the language on the waterway=pressurised page to use
standard English syntax, and to clarify that these are artificial
tunnels or pipelines, in the short description.

This was already somewhat clear from the full description, and from
the proposal where waterway=pressurised was described as "tag [for]
any pipe flow feature like water tunnels, penstocks and siphons where
water flows without any air. "

New short description:

"An artificial tunnel or pipeline where water flows in a closed space
without air"

New long description:

"This key is intended to map artificial conduits where water is found
pipe flowing in a closed space without air. As water is filling all
space available in the conduit, it applies a positive static pressure
on the conduit walls.

"The conduit can be either a pipeline or a tunnel. This is specified
by adding man_made=pipeline or tunnel=* to the way tagged
waterway=pressurised."

"A "pipe flow regime" is one of the three main flow regimes of liquid
water. This regime is obtained by design when the intake of the
conduit is built below the lowest water level the source can reach.
This is the only use case of waterway=pressurised.""

I removed the reference to natural caves, because it conflicted with
the last paragraph which says "This regime is obtained by design when
the intake of the conduit is built below the lowest water level the
source can reach" - clearly natural caves are not designed or built in
the usual sense.

The original proposal page had a brief mention of caves but said -
confusingly - that this usage was "out of scope" of the proposal: "it
may be useful for speleologists and natural underground rivers/siphons
out of this proposal scope."