Add doc for code indentation (#3799)

docs/cn/documentation.md

@@ -38,6 +38,7 @@ lang: zh
   - [快捷键导航](#快捷键导航)
   - [基本编辑操作](#基本编辑操作)
     - [移动文本块](#移动文本块)
+    - [代码缩进](#代码缩进)
     - [文本操作命令](#文本操作命令)
     - [文本插入命令](#文本插入命令)
     - [增加或减小数字](#增加或减小数字)
@@ -877,6 +878,26 @@ call SpaceVim#custom#SPC('nnoremap', ['f', 't'], 'echom "hello world"', 'test cu
 | `Ctrl-Shift-Up`   | 向上移动选中行 |
 | `Ctrl-Shift-Down` | 向下移动选中行 |
 
+#### 代码缩进
+
+默认的代码缩进值是 2，缩进的大小由选项 `default_indent` 设置，
+如果希望使用 4 个空格作为缩进，只需要在 SpaceVim 配置文件中加入如下内容：
+
+```toml
+[options]
+    default_indent = 4
+```
+
+`default_indent` 这一选项的值，将被赋值到 Vim 的选项：`&tabstop`、`&softtabstop` 和
+`&shiftwidth`。默认情况下，输入的 `<Tab>` 会被自动展开成对应缩进数量的空格，
+可通过设置选项 `expand_tab` 的值为 `false` 来禁用这一特性：
+
+```toml
+[options]
+    default_indent = 4
+    expand_tab = true
+```
+
 #### 文本操作命令
 
 文本相关的命令 (以 `x` 开头)：

docs/documentation.md

@@ -36,6 +36,7 @@ description: "General documentation about how to using SpaceVim, including the q
   - [Mappings guide](#mappings-guide)
   - [Editing](#editing)
     - [Moving text](#moving-text)
+    - [Code indentation](#code-indentation)
     - [Text manipulation commands](#text-manipulation-commands)
     - [Text insertion commands](#text-insertion-commands)
     - [Increase/Decrease numbers](#increasedecrease-numbers)
@@ -932,6 +933,27 @@ Then use `<Tab>` or `<Up>` and `<Down>` to select the mapping, press `<Enter>` t
 | `Ctrl-Shift-Up`   | move lines up                 |
 | `Ctrl-Shift-Down` | move lines down               |
 
+#### Code indentation
+
+The default indentation of code is 2, which is controlled by option `default_indent`.
+If you prefer to use 4 as code indentation. Just add following snippet into SpaceVim
+configuration file:
+
+```toml
+[options]
+    default_indent = 4
+```
+
+The `default_indent` option will be applied to vim's `&tabstop`, `&softtabstop` and
+`&shiftwidth` options. By default, when user insert a `<Tab>`, it will be expanded
+to spaces. This feature can be disabled by `expand_tab` option.
+
+```toml
+[options]
+    default_indent = 4
+    expand_tab = true
+```
+
 #### Text manipulation commands
 
 Text related commands (start with `x`):