Add note about core.upload:parallel (#3540)

Author: AbrilRBS

common/config_list.inc

@@ -25,6 +25,7 @@
     core.sources:download_urls: List of URLs to download backup sources from
     core.sources:exclude_urls: URLs which will not be backed up
     core.sources:upload_url: Remote URL to upload backup sources to
+    core.upload:parallel: Number of concurrent threads to upload packages
     core.upload:retry: Number of retries in case of failure when uploading to Conan server
     core.upload:retry_wait: Seconds to wait between upload attempts to Conan server
     core.version_ranges:resolve_prereleases: Whether version ranges can resolve to pre-releases or not

reference/commands/upload.rst

@@ -65,6 +65,9 @@ If the ``--format=json`` formatter is specified, the result will be a "PackageLi
 
 The ``--dry-run`` argument will prepare the packages for upload, zip files if necessary, check in the server to see what needs to be uploaded and what is already in the server, but it will not execute the actual upload. 
 
+Using the ``core.upload:parallel`` conf, it is possible to upload packages in parallel.
+By default, or when set to a value less than ``2``, no parallelization will take place,
+and any other value will be the number of parallel threads to utilize.
 
 Read more
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