libstdc++: Add Doxygen comment for string::resize_and_overwite

This is a complicated API that should be clearly documented.

Also improve the comment on basic_ios::_M_setstate.

libstdc++-v3/ChangeLog:

	* include/bits/basic_ios.h (basic_ios::_M_setstate): Add
	caveat to comment.
	* include/bits/basic_string.h (resize_and_overwrite): Add
	doxygen comment.

libstdc++-v3/include/bits/basic_ios.h

@@ -157,9 +157,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       setstate(iostate __state)
       { this->clear(this->rdstate() | __state); }
 
-      // Flip the internal state on for the proper state bits, then
+      // Flips the internal state on for the proper state bits, then
       // rethrows the propagated exception if bit also set in
-      // exceptions().
+      // exceptions(). Must only be called within a catch handler.
       void
       _M_setstate(iostate __state)
       {

libstdc++-v3/include/bits/basic_string.h

@@ -1117,6 +1117,35 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
 #if __cplusplus > 202002L
 #define __cpp_lib_string_resize_and_overwrite 202110L
+      /** Resize the string and call a function to fill it.
+       *
+       * @param __n   The maximum size requested.
+       * @param __op  A callable object that writes characters to the string.
+       *
+       * This is a low-level function that is easy to misuse, be careful.
+       *
+       * Calling `str.resize_and_overwrite(n, op)` will reserve at least `n`
+       * characters in `str`, evaluate `n2 = std::move(op)(str.data(), n)`,
+       * and finally set the string length to `n2` (adding a null terminator
+       * at the end). The function object `op` is allowed to write to the
+       * extra capacity added by the initial reserve operation, which is not
+       * allowed if you just call `str.reserve(n)` yourself.
+       *
+       * This can be used to efficiently fill a `string` buffer without the
+       * overhead of zero-initializing characters that will be overwritten
+       * anyway.
+       *
+       * The callable `op` must not access the string directly (only through
+       * the pointer passed as its first argument), must not write more than
+       * `n` characters to the string, must return a value no greater than `n`,
+       * and must ensure that all characters up to the returned length are
+       * valid after it returns (i.e. there must be no uninitialized values
+       * left in the string after the call, because accessing them would
+       * have undefined behaviour). If `op` exits by throwing an exception
+       * the behaviour is undefined.
+       *
+       * @since C++23
+       */
       template<typename _Operation>
 	constexpr void
 	resize_and_overwrite(size_type __n, _Operation __op);