docs: clarify custom plugin load ordering

[TyceHerrman]

Summary

• clarify that the custom-plugin convenience loader's file order is unspecified
• recommend keeping dependent plugins in the same file
• document explicit ordered requires for intentionally separate modules
• place the same warning beside the loader implementation

The loader itself remains unchanged.

Closes #2038

Testing

• stylua --check .
• git diff --check

[oriori1703]

Looks great :)