How to find and use the "specifiers"

Whilst scripting, I encounter “specificers” that are useful to acknowledge when creating a string or any type of sort, (such as string.gsub("STRING", "^%l", string.upper"), in which gives “String”), however, doing research and I couldn’t find any information on how to get these “specificiers”. Is there a page in which links to these things? I’m currently trying to find one for string.gsub(), but there’s nothing given - so my question is, how do you get them specifiers (such as the example above - ^%l being one of them)?

Try this page: