Using the word "women" as an adjective is becoming a very popular way of referring to a group of (insert noun here) who happen to be women. "Women doctors," "women firefighters," etc. You get the picture.
There are a few schools of thought on this, as a grammatical issue. There are those who believe that a two-word compound noun, when plural, only needs the actual noun to be made plural. Thus, instead of "women doctors", the plural would be "woman doctors." There are VERY few exceptions to this. Manservant/menservants is the only one I can find in any of the grammar books that I own.
However, a grammatical example has been provided, and, as promised, I'm amending this writeup. If Webster says it's right, I won't argue (the folks who sent me nasty name-calling messages will, no doubt, be thrilled).
I myself will probably continue to gravitate towards using "female doctors." It avoids the issue entirely.