Adjusted docs and makefile to use correct preproc flags

docs/API.md

@@ -84,12 +84,12 @@ of the websocket frame which URI handler can examine. Note that to
 reply to the websocket client, `mg_websocket_write()` should be used.
 To reply to the plain HTTP client, `mg_write_data()` should be used.
 
-An URI handler must return a value. If 0 is return, that signals Mongoose
+An URI handler must return a value.  If URI handler has sent all data,
+it should return `1`.  If URI handler returns `0`, that signals Mongoose
 that URI handler hasn't finished sending data to the client. In this case,
 Mongoose will call URI handler after each successful socket write.
 `struct mg_connection::wsbits` flag will indicate the status of the write,
-`1` means that write has failed and connection will be closed. If URI
-handler has sent all data, it should return `1`.
+`1` means that write has failed and connection will be closed.
 
     void mg_set_http_error_handler(struct mg_server *, mg_handler_t);
 
@@ -141,7 +141,13 @@ a response body.  Mongoose provides functions for all three parts:
    * `mg_send_data()` and `mg_printf_data()` are used to send data to the
      client. Note that Mongoose adds `Transfer-Encoding: chunked` header
      implicitly, and sends data in chunks. Therefore, it is not necessary to
-     set `Content-Length` header.
+     set `Content-Length` header. Note that `mg_send_data()` and
+     `mg_printf_data()` do not send data immediately. Instead, they spool
+     data in memory, and Mongoose sends that data later after URI handler
+     returns. If data to be sent is huge, an URI handler might
+     send data in pieces by saving state in
+     `struct mg_connection::connection_param` variable and returning `0`. Then
+     Mongoose will call a handler repeatedly after each socket write.
 
 <!-- -->
 
@@ -149,8 +155,7 @@ a response body.  Mongoose provides functions for all three parts:
                            const char *data, size_t data_len);
 
 Similar to `mg_write()`, but wraps the data into a websocket frame with a
-given websocket `opcode`. This function is available when mongoose is
-compiled with `-DUSE_WEBSOCKET`.  
+given websocket `opcode`.  
 
     const char *mg_get_header(const struct mg_connection *, const char *name);