Auto merge of #10935 - har7an:patch-1, r=epage

Mention that aliases are recursive

Instead of duplicating portions of commands that are used identically across many aliases, the user can instead reuse any previously defined aliases.

### What does this PR try to resolve?

Today I started working with [`xtask`](https://github.com/matklad/cargo-xtask/), which is a build system based on a simple cargo alias:

```toml
[alias]
xtask = "run --package xtask --"
```

Since the word "xtask" is rather difficult to type in my opinion (at least my left hand struggles quite a bit) I wanted to add another alias, `x` as a shorthand (similar to what `build`, `run`, etc. have by default). Thereby I discovered that I needn't replicate the whole alias, because aliases are recursive. I consulted the docs and couldn't find a mention of this, hence I'm adding it as part of this PR so other users can discover it.

### How should we test and review this PR?

I don't think this requires a separate test, it's a minor change to the documentation only.

### Additional information

Author: bors

src/doc/src/reference/config.md

@@ -59,6 +59,7 @@ c = "check"
 t = "test"
 r = "run"
 rr = "run --release"
+recursive_example = "rr --example recursions"
 space_example = ["run", "--release", "--", "\"command list\""]
 
 [build]
@@ -335,6 +336,14 @@ r = "run"
 
 Aliases are not allowed to redefine existing built-in commands.
 
+Aliases are recursive:
+
+```toml
+[alias]
+rr = "run --release"
+recursive_example = "rr --example recursions"
+```
+
 #### `[build]`
 
 The `[build]` table controls build-time operations and compiler settings.