explain const dependency in docs + small docs fixes

Author: 5chmidti

clang-tools-extra/docs/clang-tidy/checks/modernize/use-constexpr.rst

@@ -9,6 +9,19 @@ The check analyses any function and variable according to the rules defined
 for the language version that the code is compiled with.
 Changing to a newer language standard may therefore offer additional opportunity
 to declare a function or variable as ``constexpr``.
+Furthermore, this check can be incremental in terms of its diagnostics. For
+example, declaring a function ``constepxr`` might create new opportunities of
+marking additional variables or function ``constexpr``, which can only be found
+in subsequent runs of this check.
+
+For variables, the check will only detect variables that can be declared
+``constexpr`` if they are already ``const``.
+This is because this check would have to duplicate the expensive analysis of the
+:doc:`misc-const-correctness<clang-tidy/checks/misc/const-correctness>` check.
+Therefore, it is recommended to have 
+:doc:`misc-const-correctness<clang-tidy/checks/misc/const-correctness>` enabled
+in the Clang-Tidy config when this check is, so that all opportunities for
+``const`` and also ``constexpr`` are explored.
 
 Options
 -------
@@ -26,9 +39,9 @@ Options
       int &ref;
     };
 
-  This type is a literal type, but can not be constructed at compile-time,
-  so with `ConservativeLiteralType` equal to `true`, variables or funtions
-  with this type are not considered to support ``constexpr``. Default is
+  This type is a literal type, but can not be constructed at compile-time.
+  With `ConservativeLiteralType` equal to `true`, variables or funtions
+  with this type are not diagnosed to add ``constexpr``. Default is
   `true`.
 
 .. option:: AddConstexprToMethodOfClassWithoutConstexprConstructor
@@ -40,6 +53,12 @@ Options
 
 .. option:: ConstexprString
 
-  The string to use to specify something as ``constexpr``, for example, a macro.
-  Default is ``constexpr``.
+  The string to use to specify a variable or function as ``constexpr``, for
+  example, a macro. Default is ``constexpr``.
+
+.. option:: ConstexprString
+
+  The string to use with C++23 to specify a function-local variable as ``static
+  constexpr``, for example, a macro. Default is ``static constexpr``
+  (concatenating ``static `` with the `ConstexprString` option).