Add documentation for default index URI variable (#669)

* Add documentation for default index URI variable

* Code review changes

Author: chriskim06

site/content/docs/user-guide/configuration.md

@@ -0,0 +1,24 @@
+---
+title: Advanced Configuration
+slug: advanced-configuration
+weight: 900
+---
+
+### Use a different default index
+
+When Krew is installed, it automatically initializes an index named `default`
+pointing to the [krew-index][ki] repository. You can force Krew to use a
+different repository by setting `KREW_DEFAULT_INDEX_URI` before running the
+[installation instructions]({{<ref "setup/install.md">}}) or after [removing the
+default index]({{<ref "using-custom-indexes.md#the-default-index">}}).
+`KREW_DEFAULT_INDEX_URI` should point to a git repository URI that uses a valid
+git remote protocol.
+
+To use a different default index, set the `KREW_DEFAULT_INDEX_URI` environment
+variable in your `~/.bashrc`, `~/.bash_profile`, or `~/.zshrc`:
+
+```shell
+export KREW_DEFAULT_INDEX_URI='[email protected]:foo/custom-index.git'
+```
+
+[ki]: https://github.com/kubernetes-sigs/krew-index

site/content/docs/user-guide/setup/updates.md

@@ -10,10 +10,10 @@ upgrade to a newer version.
 This is done by calling the GitHub API, and we do not collect any data from your
 machine during this process.
 
-If you want to disable the update checks, set, the `KREW_NO_UPGRADE_CHECK`
+If you want to disable the update checks, set the `KREW_NO_UPGRADE_CHECK`
 environment variable. To permanently disable this, add the following to your
 `~/.bashrc`, `~/.bash_profile`, or `~/.zshrc`:
 
-```text
+```shell
 export KREW_NO_UPGRADE_CHECK=1
 ```