Are there any resources, documentation, where the invoke instruction (FASM) would be described?

Question

assembler

And one more thing: why does this code end up outputting "Hello worldj" instead of the adequate output "Hello world"?

include 'win32ax.inc'
MsgText db "Hello world"
start:
         invoke  MessageBox,NULL,MsgText,'Fasm message box:',MB_OK
         invoke  ExitProcess,0
.end start

Answer 1

There is no zero at the end of the string.
And invoke is not an instruction, but a macro that expands into pushing the listed parameters onto the stack (push) and calling the function.