docs: document git.fetch and git.push config options

Someone on Discord asked this exact question, but these options apparently
weren't documented when they were added in the `0.7.0` release.

Signed-off-by: Austin Seipp <aseipp@pobox.com>

docs/config.md

@@ -514,6 +514,29 @@ conflict is considered fully resolved when there are no conflict markers left.
 
 ## Git settings
 
+### Default remotes for `jj git fetch` and `jj git push`
+
+By default, if a single remote exists it is used for `jj git fetch` and `jj git
+push`; however if multiple remotes exist, the default remote is assumed to be
+named `"origin"`, just like in Git. Sometimes this is undesirable, e.g. when you
+want to fetch from a different remote than you push to, such as a GitHub fork.
+
+To change this behavior, you can modify the [repository
+configuration](#config-files-and-toml) variable `git.fetch`, which can be a
+single remote, or a list of remotes to fetch from multiple places:
+
+```
+jj config set --repo git.fetch "upstream"
+jj config set --repo git.fetch '["origin", "upstream"]'
+```
+
+Similarly, you can also set the variable `git.push` to cause `jj git push` to
+push to a different remote:
+
+```
+jj config set --repo git.push "github"
+```
+
 ### Automatic local branch creation
 
 When `jj` imports a new remote-tracking branch from Git, it can also create a

docs/config.toml

@@ -27,3 +27,7 @@ template-aliases.'format_timestamp(timestamp)' = 'timestamp.ago()'
 # see documentation in config.md. An example:
 
 # merge-tools.meld.program = "C:\\Program Files\\Meld\\meld.exe" # If not in PATH
+
+# Change the default push/fetch remote for `jj git push` and `jj git fetch`
+# git.fetch = "upstream"
+# git.push = "myfork"