chore: improve --port help text (#1346)

# What does this PR do?
[Provide a short summary of what this PR does and why. Link to relevant
issues if applicable.]

It would be better to tell user env var usage in help text.
```
before:
$ llama stack run --help
  --port PORT           Port to run the server on. Defaults to 8321

after
$ llama stack run --help
  --port PORT           Port to run the server on. It can also be passed via the env var LLAMA_STACK_PORT. Defaults to 8321
```

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan
[Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.*]

[//]: # (## Documentation)

Signed-off-by: reidliu <reid201711@gmail.com>
Co-authored-by: reidliu <reid201711@gmail.com>

docs/source/distributions/building_distro.md

@@ -194,7 +194,7 @@ positional arguments:
 
 options:
   -h, --help            show this help message and exit
-  --port PORT           Port to run the server on. Defaults to 8321
+  --port PORT           Port to run the server on. It can also be passed via the env var LLAMA_STACK_PORT. Defaults to 8321
   --image-name IMAGE_NAME
                         Name of the image to run. Defaults to the current conda environment
   --disable-ipv6        Disable IPv6 support

llama_stack/cli/stack/run.py

@@ -37,7 +37,7 @@ class StackRun(Subcommand):
         self.parser.add_argument(
             "--port",
             type=int,
-            help="Port to run the server on. Defaults to 8321",
+            help="Port to run the server on. It can also be passed via the env var LLAMA_STACK_PORT. Defaults to 8321",
             default=int(os.getenv("LLAMA_STACK_PORT", 8321)),
         )
         self.parser.add_argument(