convert : clarify sentence-transformers-dense-modules help [no ci] (#18662)
* convert : clarify sentence-transformers-dense-modules help [no ci]
This commit updates this options help message which currently looks
like this:
```console
--sentence-transformers-dense-modules
Whether to include sentence-transformers dense modules.It can be used for sentence-transformers models, like
google/embeddinggemma-300mDefault these modules are not included.
```