defaults: enable 'hlsearch' by default. #2859

Also update the documentation regarding the option.

Re: https://github.com/neovim/neovim/issues/2676

runtime/doc/options.txt

@@ -3539,7 +3539,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	See |highlight-default| for the default highlight groups.
 
 				 *'hlsearch'* *'hls'* *'nohlsearch'* *'nohls'*
-'hlsearch' 'hls'	boolean	(default off)
+'hlsearch' 'hls'	boolean	(default on)
 			global
 			{not available when compiled without the
 			|+extra_search| feature}

runtime/doc/usr_03.txt

@@ -426,18 +426,19 @@ HIGHLIGHTING MATCHES
 While editing a program you see a variable called "nr".  You want to check
 where it's used.  You could move the cursor to "nr" and use the "*" command
 and press "n" to go along all the matches.
-   There is another way.  Type this command: >
 
-	:set hlsearch
+Vim will highlight all matches. That is a very good way to see where the
-
+variable is used, without the need to type commands.
-If you now search for "nr", Vim will highlight all matches.  That is a very
-good way to see where the variable is used, without the need to type commands.
    To switch this off: >
 
 	:set nohlsearch
 
 Then you need to switch it on again if you want to use it for the next search
-command.  If you only want to remove the highlighting, use this command: >
+command: >
+
+	:set hlsearch
+
+If you only want to remove the highlighting, use this command: >
 
 	:nohlsearch
 

runtime/doc/usr_26.txt

@@ -64,8 +64,6 @@ Now press CTRL-A.  The year will be increased by one:
 
 Use "n" to find the next year, and press "." to repeat the CTRL-A ("." is a
 bit quicker to type).  Repeat "n" and "." for all years that appear.
-   Hint: set the 'hlsearch' option to see the matches you are going to change,
-then you can look ahead and do it faster.
 
 Adding more than one can be done by prepending the number to CTRL-A.  Suppose
 you have this list:

runtime/doc/vim_diff.txt

@@ -34,6 +34,7 @@ these differences.
 - 'complete' doesn't include "i"
 - 'encoding' defaults to "utf-8"
 - 'formatoptions' defaults to "tcqj"
+- 'hlsearch' is set by default
 - 'incsearch' is set by default
 - 'langnoremap' is set by default
 - 'mouse' defaults to "a"

src/nvim/option.c

@@ -880,9 +880,9 @@ static vimoption_T
   {"hkmapp",      "hkp",  P_BOOL|P_VI_DEF|P_VIM,
    (char_u *)&p_hkmapp, PV_NONE,
    {(char_u *)FALSE, (char_u *)0L} SCRIPTID_INIT},
-  {"hlsearch",    "hls",  P_BOOL|P_VI_DEF|P_VIM|P_RALL,
+  {"hlsearch",    "hls",  P_BOOL|P_VIM|P_RALL,
    (char_u *)&p_hls, PV_NONE,
-   {(char_u *)FALSE, (char_u *)0L} SCRIPTID_INIT},
+   {(char_u *)FALSE, (char_u *)TRUE} SCRIPTID_INIT},
   {"icon",        NULL,   P_BOOL|P_VI_DEF,
    (char_u *)&p_icon, PV_NONE,
    {(char_u *)FALSE, (char_u *)0L} SCRIPTID_INIT},

test/functional/ui/searchhl_spec.lua

@@ -25,7 +25,8 @@ describe('search highlighting', function()
     screen:set_default_attr_ignore( {{bold=true, foreground=hl_colors.NonText}} )
   end)
 
-  it('is off by default', function()
+  it('is disabled by ":set nohlsearch"', function()
+    execute('set nohlsearch')
     insert("some text\nmore text")
     feed("gg/text<cr>")
     screen:expect([[
@@ -40,13 +41,14 @@ describe('search highlighting', function()
   end)
 
   it('works', function()
-    execute('set hlsearch')
     insert([[
       some text
       more textstuff
       stupidtexttextstuff
       a text word
     ]])
+
+    -- 'hlsearch' is enabled by default. #2859
     feed("gg/text<cr>")
     screen:expect([[
         some {1:^text}                             |