Multiple folders in the home folder ~/ have what Finder reports as "You have custom access".
Finder's Get Info only reports simple Unix style permissions based on read/write for Owner, Group (by default Staff) and World (Everyone).
But modern file/folder permissions can also use Access Control Lists (ACL). These first appeared on Windows with Windows NT and on OS X from 10.4. But their history goes back further - certainly to VAX/VMS and RSX-11M, and Wikipedia says Multics. An ACL consists of one or more Access Control Entries (ACE).
macOS adds an ACL to some folders in `~/'. AFAIK, Apple does not provide a GUI interface for viewing or modifying ACLs.
To see the full permissions including the ACL, open Terminal and use ls commands like these two:
ls -led ~/
drwxr-x---+ 86 gilby staff 2752 7 Aug 11:17 /Users/gilby/
0: group:everyone deny delete
ls -led ~/Documents
drwx------@ 43 gilby staff 1376 18 Jul 13:34 Documents
0: group:everyone deny delete
1: group:com.apple.sharepoint.group.16 allow search
The rwxr-x--- and rwx------ are Unix style permissions for owner, group and world.
The subsequent lines are ACEs.
For both the examples the ACL it is saying that no-one (not even the user) can delete the directory.
The second ACE for ~/Documents is related to file sharing - it may not be present on your Documents folder.
I recommend you do not try to modify these or any other ACLs created by macOS. If you are determined to do so the chmod command can be used. Read man chmod for how to do this - the example commands are useful.
More safely, you can use chmod to modify the Unix style permissions - read the man page for usage and examples.
As an alternative to the command line, you can use TinkerTool System (not free) to view and modify both ACLs and Unix/POSIX permissions.
Apple's reference for file and folder permissions is File system Details which discusses both Unix (also referred to as POSIX) permissions and ACLs.
See also this question and answer What are all the available ACL attributes in Mac OS 10.13 High Sierra?.