Deprecate Error::description for real

`description` has been documented as soft-deprecated since 1.27.0 (17
months ago). There is no longer any reason to call it or implement it.

This commit:

- adds #[rustc_deprecated(since = "1.41.0")] to Error::description;

- moves description (and cause, which is also deprecated) below the
  source and backtrace methods in the Error trait;

- reduces documentation of description and cause to take up much less
  vertical real estate in rustdocs, while preserving the example that
  shows how to render errors without needing to call description;

- removes the description function of all *currently unstable* Error
  impls in the standard library;

- marks #[allow(deprecated)] the description function of all *stable*
  Error impls in the standard library;

- replaces miscellaneous uses of description in example code and the
  compiler.

2 files changed, 2 insertions, 0 deletions

diff --git a/src/libstd/sys/sgx/net.rs b/src/libstd/sys/sgx/net.rs
index f36687b4d3d..bd0652ab464 100644
--- a/src/libstd/sys/sgx/net.rs
+++ b/src/libstd/sys/sgx/net.rs
@@ -440,6 +440,7 @@ pub struct NonIpSockAddr {
 }
 
 impl error::Error for NonIpSockAddr {
+    #[allow(deprecated)]
     fn description(&self) -> &str {
         "Failed to convert address to SocketAddr"
     }
diff --git a/src/libstd/sys/sgx/os.rs b/src/libstd/sys/sgx/os.rs
index 2c5b3134219..6ed7a2f2044 100644
--- a/src/libstd/sys/sgx/os.rs
+++ b/src/libstd/sys/sgx/os.rs
@@ -66,6 +66,7 @@ impl fmt::Display for JoinPathsError {
 }
 
 impl StdError for JoinPathsError {
+    #[allow(deprecated)]
     fn description(&self) -> &str {
         "not supported in SGX yet"
     }