Export of internal Abseil changes

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44ccc0320ffaa2106ba3c6393b5a40c3b4f7b901 by Abseil Team <absl-team@google.com>:

Clarify span iterator documentation.

PiperOrigin-RevId: 299110584

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80d016d8026b8d6904aa0ff2d5e1c3ae27f129bb by Greg Falcon <gfalcon@google.com>:

Add Cord::TryFlat().

PiperOrigin-RevId: 298889772

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da6900203f1e4131d5693cbca157b6dba099bbed by Greg Falcon <gfalcon@google.com>:

clang-format cord_test.cc.

PiperOrigin-RevId: 298851425
GitOrigin-RevId: 44ccc0320ffaa2106ba3c6393b5a40c3b4f7b901
Change-Id: Ia5394f6fbb473d206726fdd48a00eb07a6acad6a

absl/types/span.h

@@ -292,60 +292,74 @@ class Span {
 
   // Span::front()
   //
-  // Returns a reference to the first element of this span.
+  // Returns a reference to the first element of this span. The span must not
+  // be empty.
   constexpr reference front() const noexcept {
     return ABSL_ASSERT(size() > 0), *data();
   }
 
   // Span::back()
   //
-  // Returns a reference to the last element of this span.
+  // Returns a reference to the last element of this span. The span must not
+  // be empty.
   constexpr reference back() const noexcept {
     return ABSL_ASSERT(size() > 0), *(data() + size() - 1);
   }
 
   // Span::begin()
   //
-  // Returns an iterator to the first element of this span.
+  // Returns an iterator pointing to the first element of this span, or `end()`
+  // if the span is empty.
   constexpr iterator begin() const noexcept { return data(); }
 
   // Span::cbegin()
   //
-  // Returns a const iterator to the first element of this span.
+  // Returns a const iterator pointing to the first element of this span, or
+  // `end()` if the span is empty.
   constexpr const_iterator cbegin() const noexcept { return begin(); }
 
   // Span::end()
   //
-  // Returns an iterator to the last element of this span.
+  // Returns an iterator pointing just beyond the last element at the
+  // end of this span. This iterator acts as a placeholder; attempting to
+  // access it results in undefined behavior.
   constexpr iterator end() const noexcept { return data() + size(); }
 
   // Span::cend()
   //
-  // Returns a const iterator to the last element of this span.
+  // Returns a const iterator pointing just beyond the last element at the
+  // end of this span. This iterator acts as a placeholder; attempting to
+  // access it results in undefined behavior.
   constexpr const_iterator cend() const noexcept { return end(); }
 
   // Span::rbegin()
   //
-  // Returns a reverse iterator starting at the last element of this span.
+  // Returns a reverse iterator pointing to the last element at the end of this
+  // span, or `rend()` if the span is empty.
   constexpr reverse_iterator rbegin() const noexcept {
     return reverse_iterator(end());
   }
 
   // Span::crbegin()
   //
-  // Returns a reverse const iterator starting at the last element of this span.
+  // Returns a const reverse iterator pointing to the last element at the end of
+  // this span, or `crend()` if the span is empty.
   constexpr const_reverse_iterator crbegin() const noexcept { return rbegin(); }
 
   // Span::rend()
   //
-  // Returns a reverse iterator starting at the first element of this span.
+  // Returns a reverse iterator pointing just before the first element
+  // at the beginning of this span. This pointer acts as a placeholder;
+  // attempting to access its element results in undefined behavior.
   constexpr reverse_iterator rend() const noexcept {
     return reverse_iterator(begin());
   }
 
   // Span::crend()
   //
-  // Returns a reverse iterator starting at the first element of this span.
+  // Returns a reverse const iterator pointing just before the first element
+  // at the beginning of this span. This pointer acts as a placeholder;
+  // attempting to access its element results in undefined behavior.
   constexpr const_reverse_iterator crend() const noexcept { return rend(); }
 
   // Span mutations