The search function on the MAGMA website is pretty terrible. It is better to use google with the "site:..." function. E.g. try
in google, and compare with the result of entering the word p-core in the magma search.
Apart from that, the only way I know of learning magma is from people who already know it and are good at it. In your magma installation, you can view a lot of the source code, and a lot of it is itself written in magma (the very core is written in C). Apart from that, try to find somebody who uses magma on a regular basis and who you can talk to whenever you have a specific question.