in manual

If --syncdelay is 0, mongod flushes all operations to disk immediately

The above statement is incorrect

These are correct:
program option

seconds between disk syncs (0=never, but not recommended)

db.cpp

warning: --syncdelay 0 is not recommended and can have strange performance

notes

• msync() was never called when syncdelay is 0.
• MemoryMappedFile::flushAll() was skipped by the line of

  if ( cmdLine.syncdelay == 0 ) 
     // skip flush calls