Update documentation

Update vim_diff.txt with :lmap differences, update documentation on
'keymap', and add tests.

The tests added are to demonstrate the behaviour specified in the
documentation of :loadkeymap.

test/functional/options/keymap_spec.lua

@@ -215,4 +215,19 @@ describe("'keymap' / :lmap", function()
     feed('il')
     expect('aalllaaa')
   end)
+  it('does not cause recursive mappings', function()
+    command('lmap a l')
+    feed('qaila<esc>q')
+    expect('allllaaa')
+    feed('u@a')
+    expect('allllaaa')
+  end)
+  it('can handle multicharacter mappings', function()
+    command("lmap 'a x")
+    command("lmap '' '")
+    feed("qai'a''a<esc>q")
+    expect("x'alllaaa")
+    feed('u@a')
+    expect("x'alllaaa")
+  end)
 end)