Added complete call-seq documentation for proc binding functionality across
both source and test files:
## Core Implementation (src/proc_binding.c):
### Main Method:
- binding: returns Binding object capturing proc's execution context,
includes comprehensive examples showing local variable access, parameter
binding, and scope retention with practical usage patterns
### Helper Functions:
- mrb_mruby_proc_binding_gem_init: initializes gem by adding binding method
to Proc class, explains method signature and return type behavior
Co-authored-by: Atlassian Rovo Dev
Yukihiro "Matz" Matsumoto