docs: Added basic description to CustomItems

[z5tm]

Description

Added a short description to CustomItems

What is the current behavior? (You can also link to an open issue here)
No description for CustomItems

What is the new behavior? (if this is a feature change)
Added a short description for CustomItems

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
No

Other information:
N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentations

Submission checklist

• I have checked the project can be compiled
• I have tested my changes and it worked as expected

Patches (if there are any changes related to Harmony patches)

• I have checked no IL patching errors in the console

Other

• Still requires more testing

[z5tm]

oops didn't mean to unassign myself