Fix mongoose docs generation

PUBLISHED_FROM=331821dcd1f7dc8a94581cd8a9b51aa00a89fddc

docs/c-api/mg_util.h/mg_base64_decode.md

@@ -0,0 +1,17 @@
+---
+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);
+---
+
+Decodes base64-encoded string `s`, `len` into the destination `dst`.
+The destination has to have enough space to hold the decoded buffer.
+Decoding stops either when all strings have been decoded or invalid an
+character appeared.
+Destination is '\0'-terminated.
+Returns the number of decoded characters. On success, that should be equal
+to `len`. On error (invalid character) the return value is smaller then
+`len`. 
+