I'm pretty sure I've seen in Jekyll blog projects with multiple source document directories, such as _posts and _pages, but the source parameter in the _config.yml file can only take 1 directory as its argument, and neither an array nor a space separated string of directories works.
Am I misunderstanding the meaning of the source parameter? I'm expecting it to be used by watch to specify which files' changes will trigger a build, and which files to build.
Also, I have fragments such as about.md which can be included in other pages. What is the best location for files like this one?
The source configuration refers to your <project_root>, not individual directories within the project root. By default, its set to your current_directory (the location from where you are running jekyll build (or) serve.
Jekyll watches all nested files and directories deep within the source directory by default.
about.md is not meant to be seen as a fragment to be included in other files. Its a full-blown "page" that would render into _site/about.html or _site/about/index.html depending on your permalink settings.
Fragments to be included in other pages live inside the _includes directory and are inserted via the Liquid construct {% include <fragment-filename>.html %}
Other than _layouts, _includes and _sass, directories that start with an underscore are ignored by Jekyll unless you configure Jekyll to see them as "collections". _posts is a pre-defined and hard-coded collection directory.
For more information on Jekyll, refer the official documentation at https://jekyllrb.com