documentation form - please report any web browser crashes

[Tom Humphrey]

I've altered the rate at which documentation form tries to cache herbarium sheet image tiles. Hopefully this will improve the speed at which images load and can be zoomed (without holes appearing).

In the past, when I tried a similar change, there were some problems with crashing older web browsers. I think I have worked around the issues, which only applied to Internet Explorer 6 and earlier, but if anyone experiences web browser crashes while trying to document sheets then please let me know and I'll revert to the earlier, less aggressive version of the documentation form.