Following is the closest thing it has to documentation, a copy of the Author's Description from the 5k entry page.
For compatibility with many existing Brainfuck programs, this interpreter limits the values of variables to the range -128 to 127 by default. To remove this restriction, enable "Large variables".
If a program attempts to read input when no more remains in the "input" field, the value zero will be used. If you want instead to be prompted to supply more input or abort the program, enable "Prompt for input".
If a program attempts to increment or decrement the pointer or a value beyond their limits, they will wrap around.
This interpreter is essentially an original work, but it could not have been written without reference to the excellent web pages linked in the "Learn more" section. In particular, the sample quine program is that written by Brian Raiter.
If you want to modify this, you may find it helpful to consult the symbols file, which indicates semi-logical associations for all those single letters used as variables and controls.
And for testing purposes, or just as a bonus, here's a little tool that will take a string of text and produce brainfuck code that outputs that string.