net: improve documentation of netgo/netcgo build tags

Updates #69680

Change-Id: If80d43da493cb035164ede4a2cadcbec5831feaf
GitHub-Last-Rev: b3853e9
GitHub-Pull-Request: #69682
Reviewed-on: https://go-review.googlesource.com/c/go/+/616262
Auto-Submit: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: mateusz834

src/net/net.go

@@ -68,6 +68,11 @@ GODEBUG environment variable (see package runtime) to go or cgo, as in:
 
 The decision can also be forced while building the Go source tree
 by setting the netgo or netcgo build tag.
+The netgo build tag disables entirely the use of the native (CGO) resolver,
+meaning the Go resolver is the only one that can be used.
+With the netcgo build tag the native and the pure Go resolver are compiled into the binary,
+but the native (CGO) resolver is preferred over the Go resolver.
+With netcgo, the Go resolver can still be forced at runtime with GODEBUG=netdns=go.
 
 A numeric netdns setting, as in GODEBUG=netdns=1, causes the resolver
 to print debugging information about its decisions.