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_base64_decode.md

@@ -0,0 +1,16 @@
+---
+title: "mg_base64_decode()"
+decl_name: "mg_base64_decode"
+symbol_kind: "func"
+signature: |
+  int mg_base64_decode(const unsigned char *s, int len, char *dst);
+---
+
+Decode base64-encoded string `s`, `len` into the destination `dst`.
+Destination has to have enough space to hold decoded buffer.
+Decoding stops either when all string has been decoded, or invalid
+character appeared.
+Destination is '\0'-terminated.
+Return number of decoded characters. On success, that should be equal to
+`len`. On error (invalid character) the return value is smaller then `len`. 
+