Rollup merge of #142102 - kiseitai3:141714_stdin_read_to_string_docs, r=tgross35

docs: Small clarification on the usage of read_to_string and read_to_end trait methods

Small clarification on the usage of read_to_string and read_to_end trait methods. The goal is to make it clear that these trait methods will become locked up if attempting to read to the end of stdin (which is a bit non-sensical unless the other end closes the pipe).

Fixes: rust-lang/rust#141714

Author: fmease