Fix optionality of safe/unsafe in guide section

The 2023-10-11 consensus from lang on how to move forward included
that the `safe` and `unsafe` keywords would be optional within an
`unsafe extern` block.

The reference-level section correctly followed this consensus, but the
guide-level section did not, and suggested that annotating items with
these keywords was required.  Let's fix that.

Author: traviscross

text/0000-unsafe-extern-blocks.md

@@ -30,7 +30,7 @@ An `extern` block can be placed anywhere a function declaration could appear (ge
 
 Within an `extern` block is zero or more declarations of external functions and/or external static values.  An extern function is declared with a `;` instead of a function body (similar to a method of a trait).  An extern static value is also declared with a `;` instead of an expression (similar to an associated const of a trait).  In both cases, the actual function body or value is provided by whatever external source (which is probably not even written in Rust).
 
-When an `unsafe extern` block is used, all declarations within that `extern` block *must* have the `unsafe` or `safe` keywords as part of their signature.  The `safe` keyword is a contextual keyword; it is currently allowed only within `extern` blocks.
+Declarations within an `unsafe extern` block *may* annotate their signatures with either `safe` or `unsafe`.  If a signature within the block is not annotated, it is assumed to be `unsafe`.  The `safe` keyword is contextual and is currently allowed only within `extern` blocks.
 
 If an `extern` block is used in an older edition without the `unsafe` keyword, declarations *cannot* specify `safe` or `unsafe`.  Code must update to `unsafe extern` style blocks if it wants to make `safe` declarations.