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)