Document "smart tab" option

Author: carlobaldassi

README.md

@@ -48,6 +48,10 @@ is found (e.g. `\al`), a list of possible completions is suggested (e.g. `\aleph
 `\alpha`), and it will be refined while you enter more characters; when only one match is left, pressing
 <kbd>Tab</kbd> will complete it and pressing it again will perform the substitution to Unicode.
 
+If you activate a "smart" mode by setting `g:latex_to_unicode_smart_tab = 1`, then you will also be
+able to navigate the menu of suggestions with <kbd>Tab</kbd> and <kbd>SHIFT-Tab</kbd>, and to accept your
+choice with <kbd>Enter</kbd>, similarly to other completion plugins.
+
 If no suitable substitution is found, the action will fall back to whatever mapping was previously
 defined: by default, inserting a literal `<Tab>` character, or invoking some other action if another
 plug-in is installed, e.g. [supertab] or [YouCompleteMe].

doc/julia-vim-L2U.txt

@@ -142,6 +142,11 @@ match for `\nequiv` (`≢`). By default, if <Tab> finds an exact match it perfor
 the substitution, but this can be controlled by the |g:latex_to_unicode_eager|
 setting.
 
+An additional option allows to configure the plugin to work in a more
+"natural" way, similarly to other completion plug-ins, so that when a menu is
+present it can be navigated with <Tab> and <S-Tab>, and entries can be
+selected with <Enter>. See |g:latex_to_unicode_smart_tab|.
+
 Command-line mode                                      *julia-vim-L2U-cmdmode*
 
 In |Command-line| mode, the behavior is largely the same except that both
@@ -285,6 +290,32 @@ g:latex_to_unicode_tab
                 while editing, but you need to invoke |LaTeXtoUnicode#Init()|
                 for the change to take effect.
 
+                                             *g:latex_to_unicode_smart_tab*
+g:latex_to_unicode_smart_tab
+
+                In "smart tab" mode, the <Tab> key can be used to scroll
+                through the list of suggestions when the menu produced by the
+                LaTex-to-Unicode plugin is present (otherwise this would be done
+                with <C-N>, see |i_CTRL-N|). This mode also remaps <S-Tab> to
+                scroll back (otherwise this would be done with <C-P>, see
+                |i_CTRL-P|) and it also remaps <Enter> to accept a suggestion
+                and perform the substitution (otherwise this would be done
+                with <Tab>).
+                In all cases, the remapped keys fall back to their original
+                behavior if no menu is present, or if it was not generated by
+                LaTeX-to-Unicode (this behavior may be the Vim default, or the
+                one from another existing mapping).
+
+                This only has an affect when `g:latex_to_unicode_tab` is
+                activated for insert mode. By default it is `0` (off).
+                You can enable the feature by default by inserting the line
+>
+                    let g:latex_to_unicode_auto = 1
+<
+                in your |.vimrc| file. You can change this setting at any
+                moment while editing, but you need to invoke
+                |LaTeXtoUnicode#Init()| for the change to take effect.
+
                                            *g:latex_to_unicode_suggestions*
 g:latex_to_unicode_suggestions