cmd/go: document where get writes, and not vendor directories

Fixes #12260.

Change-Id: I95c27aad6de8064b9a205d4ee507bce75926f16d
Reviewed-on: https://go-review.googlesource.com/17948Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/alldocs.go

@@ -497,6 +497,10 @@ missing packages but does not use it to look for updates to existing packages.
 
 Get also accepts build flags to control the installation. See 'go help build'.
 
+When checking out a new package, get creates the target directory
+GOPATH/src/<import-path>. If the GOPATH contains multiple entries,
+get uses the first one. See 'go help gopath'.
+
 When checking out or updating a package, get looks for a branch or tag
 that matches the locally installed version of Go. The most important
 rule is that if the local installation is running version "go1", get
@@ -507,6 +511,8 @@ Unless vendoring support is disabled (see 'go help gopath'),
 when go get checks out or updates a Git repository,
 it also updates any git submodules referenced by the repository.
 
+Get never checks out or updates code stored in vendor directories.
+
 For more about specifying packages, see 'go help packages'.
 
 For more about how 'go get' finds source code to

src/cmd/go/get.go

@@ -45,6 +45,10 @@ missing packages but does not use it to look for updates to existing packages.
 
 Get also accepts build flags to control the installation. See 'go help build'.
 
+When checking out a new package, get creates the target directory 
+GOPATH/src/<import-path>. If the GOPATH contains multiple entries,
+get uses the first one. See 'go help gopath'.
+
 When checking out or updating a package, get looks for a branch or tag
 that matches the locally installed version of Go. The most important
 rule is that if the local installation is running version "go1", get
@@ -55,6 +59,8 @@ Unless vendoring support is disabled (see 'go help gopath'),
 when go get checks out or updates a Git repository,
 it also updates any git submodules referenced by the repository.
 
+Get never checks out or updates code stored in vendor directories.
+
 For more about specifying packages, see 'go help packages'.
 
 For more about how 'go get' finds source code to