dlmalloc: add missing docs

[virus-rpi]

This pull request includes several documentation improvements to the dlmalloc module. The changes primarily involve adding comments to functions to describe their purpose and functionality.

Documentation improvements:

• vlib/dlmalloc/dlmalloc.v: Added a comment to the new function to describe its purpose of creating a new instance of Dlmalloc with the given system allocator.
• vlib/dlmalloc/dlmalloc.v: Added a comment to the malloc function to describe its purpose of allocating a block of memory of the given size.
• vlib/dlmalloc/dlmalloc_sys_nix.c.v: Added a comment to the get_system_allocator function to describe its purpose of returning an allocator that uses the system allocator.
• vlib/dlmalloc/dlmalloc_sys_windows.c.v: Added a comment to the get_system_allocator function to describe its purpose of returning an allocator that uses the system allocator.

[huly-for-github]

Connected to Huly®: V_0.6-22270

[spytheman]

Excellent work.