Improve easytags_auto_{update,highlight} option documentation

Try to prevent confusion like in issue #92 on GitHub by @oryband:

  I had to comment out the auto_update/highlight lines in order for
  easytags to create the local .tags file. I thought that these
  options were meant only for on_cursorhold. The documentation
  regarding this is quite ambiguous: If on_cursorhold is off,
  why does auto_update/highlight also apply to events?
  Events aren't auto - I set them manually.

For details: #92

README.md

@@ -179,12 +179,16 @@ By default the plug-in automatically updates and highlights your tags when you s
 
     :let g:easytags_auto_update = 0
 
+This disables all _automatic_ tags file updates (regardless of how they were enabled) where automatic means _initiated by a Vim automatic command_.
+
 ### The `g:easytags_auto_highlight` option
 
 By default the plug-in automatically updates and highlights your tags when you stop typing for a moment. If you want to disable automatic highlighting while keeping automatic updating enabled you can set this option to false:
 
     :let g:easytags_auto_highlight = 0
 
+This disables all _automatic_ tags highlighting (regardless of how it was enabled) where automatic means _initiated by a Vim automatic command_.
+
 ### The `g:easytags_autorecurse` option
 
 When the `:UpdateTags` command is executed automatically or without arguments, it defaults to updating just the tags for the current file. If you'd rather have it recursively scan everything below the directory of the current file then set this option to true (1):

doc/easytags.txt

@@ -399,6 +399,9 @@ keeping automatic highlighting enabled you can set this option to false:
 >
   :let g:easytags_auto_update = 0
 <
+This disables all _automatic_ tags file updates (regardless of how they were
+enabled) where automatic means _initiated by a Vim automatic command_.
+
 -------------------------------------------------------------------------------
 The *g:easytags_auto_highlight* option
 
@@ -408,6 +411,9 @@ keeping automatic updating enabled you can set this option to false:
 >
   :let g:easytags_auto_highlight = 0
 <
+This disables all _automatic_ tags highlighting (regardless of how it was
+enabled) where automatic means _initiated by a Vim automatic command_.
+
 -------------------------------------------------------------------------------
 The *g:easytags_autorecurse* option