[clang-tidy][docs] Use std::optional instead of absl::optional in exa…

…mples

The standard type is vastly more popular than the Abseil polyfill, so it
makes more sense to use it in documentation, even though the checker
actually understands both (and that fact is documented already).

clang-tools-extra/docs/clang-tidy/checks/bugprone-unchecked-optional-access.rst

@@ -42,7 +42,7 @@ existence check:
 
 .. code-block:: c++
 
-   void f(absl::optional<int> opt) {
+   void f(std::optional<int> opt) {
      use(*opt); // unsafe: it is unclear whether `opt` has a value.
    }
 
@@ -54,7 +54,7 @@ branches of the code. For example:
 
 .. code-block:: c++
 
-   void f(absl::optional<int> opt) {
+   void f(std::optional<int> opt) {
      if (opt.has_value()) {
      } else {
        use(opt.value()); // unsafe: it is clear that `opt` does *not* have a value.
@@ -99,11 +99,11 @@ optional has a value. For example:
 
 .. code-block:: c++
 
-   void g(absl::optional<int> opt) {
+   void g(std::optional<int> opt) {
      use(*opt); // unsafe: it is unclear whether `opt` has a value.
    }
 
-   void f(absl::optional<int> opt) {
+   void f(std::optional<int> opt) {
      if (opt.has_value()) {
        g(opt);
      }
@@ -121,7 +121,7 @@ example:
 
 .. code-block:: c++
 
-   void f(absl::optional<int> opt) {
+   void f(std::optional<int> opt) {
      if (opt.has_value()) {
        use(*opt);
      }
@@ -137,9 +137,9 @@ have a value. For example:
 
 .. code-block:: c++
 
-   void f(absl::optional<int> opt1) {
+   void f(std::optional<int> opt1) {
      if (opt1.has_value()) {
-       absl::optional<int> opt2 = opt1;
+       std::optional<int> opt2 = opt1;
        use(*opt2);
      }
    }
@@ -154,7 +154,7 @@ a value. For example:
 
 .. code-block:: c++
 
-   void f(absl::optional<int> opt) {
+   void f(std::optional<int> opt) {
      DCHECK(opt.has_value());
      use(*opt);
    }
@@ -168,7 +168,7 @@ paths that lead to an access. For example:
 
 .. code-block:: c++
 
-   void f(absl::optional<int> opt) {
+   void f(std::optional<int> opt) {
      bool safe = false;
      if (opt.has_value() && SomeOtherCondition()) {
        safe = true;
@@ -223,7 +223,7 @@ check the optional again in the local scope of the callee. For example:
      use(val);
    }
 
-   void f(absl::optional<int> opt) {
+   void f(std::optional<int> opt) {
      if (opt.has_value()) {
        g(*opt);
      }
@@ -234,7 +234,7 @@ and
 .. code-block:: c++
 
    struct S {
-     absl::optional<int> opt;
+     std::optional<int> opt;
      int x;
    };
 
@@ -260,7 +260,7 @@ The check is aware of aliases of optional types that are created via
 
 .. code-block:: c++
 
-   using OptionalInt = absl::optional<int>;
+   using OptionalInt = std::optional<int>;
 
    void f(OptionalInt opt) {
      use(opt.value()); // unsafe: it is unclear whether `opt` has a value.