Auto merge of #26642 - remram44:doc-openoptions-missing-write, r=alexcrichton
Auto merge of #26642 - remram44:doc-openoptions-missing-write, r=alexcrichton

Setting append without write doesn't give you a writeable file. Showing it as an example in the docs is confusing at best ([reddit](https://www.reddit.com/r/rust/comments/3bbz8w/why_is_writing_a_file_not_working_for_me/))

Using truncate (O_TRUNC) on a read-only file is an error on POSIX systems ("unspecified"). Note however that using create (O_CREAT) with read-only flags is fine.

Related: #26103 (which IMHO is wrong; saying "append is different than write" when should simply be "append needs write". My vote is to make append imply write)

mvdnes/rust,jroesch/rust,vhbit/rust,jroesch/rust,reem/rust,GBGamer/rust,dwillmer/rust,l0kod/rust,nwin/rust,pshc/rust,l0kod/rust,l0kod/rust,reem/rust,vhbit/rust,l0kod/rust,dwillmer/rust,KokaKiwi/rust,aidancully/rust,nwin/rust,KokaKiwi/rust,jashank/rust,graydon/rust,zubron/rust,pshc/rust,jashank/rust,reem/rust,graydon/rust,zubron/rust,jroesch/rust,pelmers/rust,pshc/rust,GBGamer/rust,jashank/rust,dwillmer/rust,KokaKiwi/rust,aidancully/rust,pshc/rust,reem/rust,zubron/rust,l0kod/rust,dwillmer/rust,mvdnes/rust,aidancully/rust,GBGamer/rust,pelmers/rust,mvdnes/rust,jashank/rust,dwillmer/rust,nwin/rust,dwillmer/rust,mvdnes/rust,nwin/rust,vhbit/rust,zubron/rust,KokaKiwi/rust,GBGamer/rust,reem/rust,jroesch/rust,zubron/rust,mvdnes/rust,jroesch/rust,vhbit/rust,GBGamer/rust,pshc/rust,graydon/rust,vhbit/rust,mvdnes/rust,vhbit/rust,GBGamer/rust,aidancully/rust,zubron/rust,vhbit/rust,pelmers/rust,KokaKiwi/rust,pelmers/rust,pelmers/rust,nwin/rust,jashank/rust,KokaKiwi/rust,pshc/rust,l0kod/rust,l0kod/rust,aidancully/rust,nwin/rust,pshc/rust,jashank/rust,jroesch/rust,pshc/rust,jroesch/rust,aidancully/rust,l0kod/rust,reem/rust,dwillmer/rust,jroesch/rust,pelmers/rust,jashank/rust,graydon/rust,graydon/rust,GBGamer/rust,jashank/rust,vhbit/rust,graydon/rust,nwin/rust,zubron/rust,nwin/rust,GBGamer/rust,zubron/rust,dwillmer/rust