docs: standardize format for usage

Use the usage output of `--help` for each subcommand, to make
sure all flags/options are included on the page, and to make
it easier to keep docs in sync.

Note that the usage output is only used when reading these
docs on GitHub; docs.docker.com only consumes the "description"
and "example" sections (when present), and generates flag information
and usage output from source

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/buildx_use.md

@@ -1,4 +1,15 @@
-# `buildx use NAME`
+# buildx use
+
+```
+Usage:  docker buildx use [OPTIONS] NAME
+
+Set the current builder instance
+
+Options:
+      --builder string   Override the configured builder instance
+      --default          Set builder as default for current context
+      --global           Builder persists context changes
+```
 
 ## Description