go/build: move build constraint docs to 'go help buildconstraint'
CL 228017 added a new help page 'go help buildconstraint' which
summarized the information on build constraints in the go/build
documentation. The summary was almost as long as the go/build
documentation, since there's very little that can be left out.
This CL moves the original go/build documentation to
'go help buildconstraint' to eliminate redundnancy. The text
describing enabled tags is slightly different (targeting command-line
users more than go/build users), but the rest of the documentation is
unchanged.
Fixes #37018
Change-Id: Ic0ed4c6fdae2395dd58852e1600c701247c9c4cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/232981
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Bryan C. Mills <bcmills@google.com>