Docs are generated for Mongoose

Comments in headers are changed a bit: removed adoc-specific stuff,
markdown is used instead

PUBLISHED_FROM=9242cce85cc52a47a197d377e7e23804721a6bb5

docs/c-api/util.h/mg_avprintf.md

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+---
+title: "mg_avprintf()"
+decl_name: "mg_avprintf"
+symbol_kind: "func"
+signature: |
+  int mg_avprintf(char **buf, size_t size, const char *fmt, va_list ap);
+---
+
+Print message to buffer. If buffer is large enough to hold the message,
+return buffer. If buffer is to small, allocate large enough buffer on heap,
+and return allocated buffer.
+This is a supposed use case:
+
+   char buf[5], *p = buf;
+   p = mg_avprintf(&p, sizeof(buf), "%s", "hi there");
+   use_p_somehow(p);
+   if (p != buf) {
+     free(p);
+   }
+
+The purpose of this is to avoid malloc-ing if generated strings are small. 
+