Fix #1128 - Add g:ale_linters_explicit for only enabling linters explicitly

doc/ale.txt

@@ -946,23 +946,25 @@ g:ale_linters                                                   *g:ale_linters*
   Type: |Dictionary|
   Default: `{}`
 
-  The |g:ale_linters| option sets a |Dictionary| mapping a filetype
-  to a |List| of linter programs to be run when checking particular filetypes.
-  Only the filetypes specified in the dictionary will be limited in terms
-  of which linters will be run.
+  The |g:ale_linters| option sets a |Dictionary| mapping a filetype to a
+  |List| of linter programs to be run when checking particular filetypes.
 
   This |Dictionary| will be merged with a default dictionary containing the
   following values: >
 
   {
   \   'csh': ['shell'],
+  \   'go': ['gofmt', 'golint', 'go vet'],
+  \   'help': [],
+  \   'python': ['flake8', 'mypy', 'pylint'],
   \   'rust': ['cargo'],
+  \   'spec': [],
   \   'text': [],
   \   'zsh': ['shell'],
   \}
 <
   This option can be used to enable only a particular set of linters for a
-  file. For example, you can enable only 'eslint' for JavaScript files: >
+  file. For example, you can enable only `eslint` for JavaScript files: >
 
   let g:ale_linters = {'javascript': ['eslint']}
 <
@@ -971,14 +973,15 @@ g:ale_linters                                                   *g:ale_linters*
 
   let g:ale_linters = {'javascript': []}
 <
-  All linters available for a given filetype can be enabled by using the
-  string `'all'`: >
+  All linters will be run for unspecified filetypes. All available linters can
+  be enabled explicitly for a given filetype by passing the string `'all'`,
+  instead of a List. >
 
   let g:ale_linters = {'c': 'all'}
 <
   Linters can be configured in each buffer with buffer-local variables. ALE
   will first look for linters for filetypes in the `b:ale_linters` variable,
-  then `g:ale_linters`, and then a default Dictionary.
+  then `g:ale_linters`, and then the default Dictionary mentioned above.
 
   `b:ale_linters` can be set to a List, or the string `'all'`. When linters
   for two different filetypes share the same name, the first linter loaded
@@ -994,6 +997,19 @@ g:ale_linters                                                   *g:ale_linters*
   " Explicitly enable all available linters for the filetype.
   let b:ale_linters = 'all'
 <
+  ALE can be configured to disable all linters unless otherwise specified with
+  `g:ale_enabled` or `b:ale_enabled` with the option |g:ale_linters_explicit|.
+
+
+g:ale_linters_explicit                                 *g:ale_linters_explicit*
+
+  Type: |Number|
+  Default: `0`
+
+  When set to `1`, only the linters from |g:ale_linters| and |b:ale_linters|
+  will be enabled. The default behavior for ALE is to enable as many linters
+  as possible, unless otherwise specified.
+
 
 g:ale_loclist_msg_format                             *g:ale_loclist_msg_format*