Add mbuf_clear()

PUBLISHED_FROM=754b98b3000b24ad3b3d9cc0b4c480ca005a1c3f

docs/c-api/mbuf.h/intro.md

@@ -4,6 +4,7 @@ symbol_kind: "intro"
 decl_name: "mbuf.h"
 items:
   - { name: mbuf_append.md }
+  - { name: mbuf_clear.md }
   - { name: mbuf_free.md }
   - { name: mbuf_init.md }
   - { name: mbuf_insert.md }

docs/c-api/mbuf.h/mbuf_clear.md

@@ -0,0 +1,10 @@
+---
+title: "mbuf_clear()"
+decl_name: "mbuf_clear"
+symbol_kind: "func"
+signature: |
+  void mbuf_clear(struct mbuf *);
+---
+
+Removes all the data from mbuf (if any). 
+