Enable gopls by default with GOPATH detection

doc/ale-go.txt

@@ -194,12 +194,30 @@ g:ale_go_gometalinter_lint_package         *g:ale_go_gometalinter_lint_package*
 ===============================================================================
 gopls                                                            *ale-go-gopls*
 
+gopls is the official Go language server, and is enabled for use with ALE by
+default.
+
+To install the latest stable version of `gopls` to your `$GOPATH`, try the
+following command: >
+
+  GO111MODULE=on go get golang.org/x/tools/gopls@latest
+<
+If `$GOPATH` is readable by ALE, it should probably work without you having to
+do anything else. See the `gopls` README file for more information:
+
+https://github.com/golang/tools/blob/master/gopls/README.md
+
+
 g:ale_go_gopls_executable                           *g:ale_go_gopls_executable*
                                                     *b:ale_go_gopls_executable*
   Type: |String|
   Default: `'gopls'`
 
-  Location of the gopls binary file.
+  See |ale-integrations-local-executables|
+
+  ALE will search for `gopls` in locally installed directories first by
+  default, and fall back on a globally installed `gopls` if it can't be found
+  otherwise.
 
 
 g:ale_go_gopls_options                                 *g:ale_go_gopls_options*
@@ -229,6 +247,15 @@ g:ale_go_gopls_init_options                       *g:ale_go_gopls_init_options*
   For a full list of supported analyzers, see:
   https://github.com/golang/tools/blob/master/gopls/doc/analyzers.md
 
+
+g:ale_go_gopls_use_global                           *g:ale_go_gopls_use_global*
+                                                    *b:ale_go_gopls_use_global*
+  Type: |String|
+  Default: `get(g:, 'ale_use_global_executables', 0)`
+
+  See |ale-integrations-local-executables|
+
+
 ===============================================================================
 govet                                                            *ale-go-govet*