document issue with FormRequest.from_response due to bug in lxml
document issue with FormRequest.from_response due to bug in lxml

This can make the spider fail due to incorrect values being posted
server-side, which is extremely hard to debug because it is easy
to miss leading/trailing whitespace, even with a logging proxy.

The fix was merged for lxml 3.8 in lxml/lxml#228 so document that
as well.

starrify/scrapy,finfish/scrapy,taito/scrapy,dangra/scrapy,pawelmhm/scrapy,scrapy/scrapy,pablohoffman/scrapy,elacuesta/scrapy,Ryezhang/scrapy,ArturGaspar/scrapy,pablohoffman/scrapy,scrapy/scrapy,rolando/scrapy,kmike/scrapy,umrashrf/scrapy,shaform/scrapy,pawelmhm/scrapy,taito/scrapy,Parlin-Galanodel/scrapy,rolando-contrib/scrapy,pawelmhm/scrapy,wujuguang/scrapy,Ryezhang/scrapy,finfish/scrapy,eLRuLL/scrapy,elacuesta/scrapy,ArturGaspar/scrapy,umrashrf/scrapy,rolando/scrapy,eLRuLL/scrapy,shaform/scrapy,kmike/scrapy,Ryezhang/scrapy,eLRuLL/scrapy,rolando/scrapy,kmike/scrapy,pablohoffman/scrapy,dangra/scrapy,dangra/scrapy,Parlin-Galanodel/scrapy,starrify/scrapy,Parlin-Galanodel/scrapy,umrashrf/scrapy,darkrho/scrapy-scrapy,rolando-contrib/scrapy,darkrho/scrapy-scrapy,ssteo/scrapy,elacuesta/scrapy,ArturGaspar/scrapy,ssteo/scrapy,wujuguang/scrapy,ssteo/scrapy,darkrho/scrapy-scrapy,scrapy/scrapy,wujuguang/scrapy,taito/scrapy,shaform/scrapy,finfish/scrapy,rolando-contrib/scrapy,starrify/scrapy