Show a single virtualtext message by default

Show only a single virtualtext message per line by default. The setting
can be configured to whatever the user wants. This default prevents
several linters from spamming the editor with messages that run off into
the right margin.

Documentation now clarifies that problems have a predictable order, and
which message will come first.

test/test_virtualtext.vader

@@ -2,6 +2,7 @@ Before:
   Save g:ale_buffer_info
   Save g:ale_virtualtext_cursor
   Save g:ale_virtualtext_delay
+  Save g:ale_virtualtext_single
   Save g:ale_virtualtext_prefix
   Save b:ale_virtualtext_prefix
   Save g:ale_use_neovim_diagnostics_api
@@ -9,7 +10,9 @@ Before:
   call ale#virtualtext#ResetDataForTests()
 
   let g:setting = ''
+  let g:ale_virtualtext_prefix = '%comment% %type%: '
   let g:ale_virtualtext_delay = 0
+  let g:ale_virtualtext_single = 0
   let g:ale_buffer_info = {
   \ bufnr(''): {
   \   'loclist': [
@@ -167,6 +170,21 @@ Execute(We should set errors across all lines):
     endif
   endif
 
+Execute(We should be able to limit virtual messages to the first one only):
+  let g:ale_virtualtext_single = 1
+
+  if has('patch-9.0.0297') || has('nvim-0.8.0')
+    call ale#virtualtext#SetTexts(bufnr(''), g:ale_buffer_info[bufnr('')].loclist)
+
+    AssertEqual '# W: Line 2 warning 1', ale#virtualtext#GetLastMessageForTests()
+
+    if has('patch-9.0.0297')
+      AssertEqual ['ALEVirtualTextError'], map(prop_list(1), {_, v -> v.type})
+      AssertEqual ['ALEVirtualTextWarning'],
+      \ map(prop_list(2), {_, v -> v.type})
+    endif
+  endif
+
 Execute(We should not set cursor messages when Neovim diagnostics are enabled):
   let g:ale_use_neovim_diagnostics_api = 1