where is the doc about @if exist for NMAKE

where is the doc about @if exist for NMAKE

I can use @if exist to test a file does exist or not, but did not find the
document via Google. For example, in my makefile for nmake ;
clean:
@if exist $(BIN_DIR) rmdir /S /Q $(BIN_DIR)
@if exist $(OBJ_DIR) rmdir /S /Q $(OBJ_DIR)