Enhanced docs of Pass Execution (If) keywords.
Enhanced docs of Pass Execution (If) keywords.

Update issue 174
Cc: pekka.klarck
Enhanced kw docs.

Also found some issues when looking at the code in BuiltIn:
- These keywords belong to _Control class.
- If-version should support non-existing variables in message and tags when
  condition is false.

ChrisHirsch/robotframework,rwarren14/robotframework,ashishdeshpande/robotframework,alexandrul-ci/robotframework,xiaokeng/robotframework,un33k/robotframework,kyle1986/robortframe,joongh/robotframework,userzimmermann/robotframework,fingeronthebutton/robotframework,kurtdawg24/robotframework,ashishdeshpande/robotframework,stasiek/robotframework,edbrannin/robotframework,userzimmermann/robotframework,snyderr/robotframework,stasiek/robotframework,suvarnaraju/robotframework,robotframework/robotframework,SivagnanamCiena/robotframework,nmrao/robotframework,alexandrul-ci/robotframework,wojciechtanski/robotframework,robotframework/robotframework,jorik041/robotframework,yonglehou/robotframework,JackNokia/robotframework,JackNokia/robotframework,alexandrul-ci/robotframework,synsun/robotframework,jaloren/robotframework,suvarnaraju/robotframework,ashishdeshpande/robotframework,rwarren14/robotframework,synsun/robotframework,wojciechtanski/robotframework,stasiek/robotframework,rwarren14/robotframework,synsun/robotframework,fingeronthebutton/robotframework,xiaokeng/robotframework,synsun/robotframework,userzimmermann/robotframework,edbrannin/robotframework,ChrisHirsch/robotframework,eric-stanley/robotframework,jaloren/robotframework,moto-timo/robotframework,ashishdeshpande/robotframework,xiaokeng/robotframework,joongh/robotframework,suvarnaraju/robotframework,HelioGuilherme66/robotframework,jaloren/robotframework,jorik041/robotframework,un33k/robotframework,rwarren14/robotframework,yonglehou/robotframework,kurtdawg24/robotframework,SivagnanamCiena/robotframework,kurtdawg24/robotframework,fingeronthebutton/robotframework,ChrisHirsch/robotframework,Colorfulstan/robotframework,fingeronthebutton/robotframework,xiaokeng/robotframework,SivagnanamCiena/robotframework,Colorfulstan/robotframework,moto-timo/robotframework,yahman72/robotframework,snyderr/robotframework,un33k/robotframework,joongh/robotframework,yonglehou/robotframework,yahman72/robotframework,alexandrul-ci/robotframework,dkentw/robotframework,jorik041/robotframework,snyderr/robotframework,suvarnaraju/robotframework,dkentw/robotframework,eric-stanley/robotframework,joongh/robotframework,dkentw/robotframework,yahman72/robotframework,wojciechtanski/robotframework,eric-stanley/robotframework,wojciechtanski/robotframework,joongh/robotframework,nmrao/robotframework,ashishdeshpande/robotframework,moto-timo/robotframework,HelioGuilherme66/robotframework,userzimmermann/robotframework,stasiek/robotframework,jaloren/robotframework,jorik041/robotframework,SivagnanamCiena/robotframework,yonglehou/robotframework,nmrao/robotframework,fingeronthebutton/robotframework,yonglehou/robotframework,moto-timo/robotframework,nmrao/robotframework,Colorfulstan/robotframework,moto-timo/robotframework,kyle1986/robortframe,un33k/robotframework,ChrisHirsch/robotframework,yahman72/robotframework,kyle1986/robortframe,edbrannin/robotframework,JackNokia/robotframework,suvarnaraju/robotframework,JackNokia/robotframework,eric-stanley/robotframework,snyderr/robotframework,rwarren14/robotframework,wojciechtanski/robotframework,stasiek/robotframework,alexandrul-ci/robotframework,Colorfulstan/robotframework,Colorfulstan/robotframework,kurtdawg24/robotframework,HelioGuilherme66/robotframework,xiaokeng/robotframework,robotframework/robotframework,kyle1986/robortframe,userzimmermann/robotframework,yahman72/robotframework,dkentw/robotframework,kyle1986/robortframe,synsun/robotframework,jaloren/robotframework,snyderr/robotframework,un33k/robotframework,nmrao/robotframework,dkentw/robotframework,ChrisHirsch/robotframework,JackNokia/robotframework,SivagnanamCiena/robotframework,jorik041/robotframework,edbrannin/robotframework,kurtdawg24/robotframework,edbrannin/robotframework