PR #2064: docs: document MSVC compile-time format checking limitation in str_format.h

Imported from GitHub PR https://github.com/abseil/abseil-cpp/pull/2064

Fixes #1478

absl::StrFormat compile-time format string checking relies on
__attribute__((format(...))), a GCC/Clang-specific compiler extension
that MSVC does not support. This adds a note to the file-level
documentation so MSVC users are aware that format string errors will
only be caught at runtime, not at compile time.
Merge 81977d79547f3a4e35dd3ce1c2ad54b1de2fcb65 into e7a10c8ec2ab4a251d1523812f10318431f1a14a

Merging this change closes #2064

COPYBARA_INTEGRATE_REVIEW=https://github.com/abseil/abseil-cpp/pull/2064 from DrishtiTripathi2230:docs/strformat-msvc-compile-time-check 81977d79547f3a4e35dd3ce1c2ad54b1de2fcb65
PiperOrigin-RevId: 922922072
Change-Id: I91f545b6a1e64f3d8aba3350ac6332cb9548a40a

absl/strings/str_format.h

diff --git a/absl/strings/str_format.h b/absl/strings/str_format.h
index ffa7f11..18a21a9 100644
--- a/absl/strings/str_format.h
+++ b/absl/strings/str_format.h
@@ -49,7 +49,8 @@
 //   * A `FormatSpec` class template fully encapsulates a format string and its
 //     type arguments and is usually provided to `str_format` functions as a
 //     variadic argument of type `FormatSpec<Arg...>`. The `FormatSpec<Args...>`
-//     template is evaluated at compile-time, providing type safety.
+//     template is evaluated at compile-time, providing type safety (supported
+//     on GCC and Clang; on MSVC, these checks are deferred to runtime).
 //   * A `ParsedFormat` instance, which encapsulates a specific, pre-compiled
 //     format string for a specific set of type(s), and which can be passed
 //     between API boundaries. (The `FormatSpec` type should not be used
@@ -275,7 +276,9 @@
 // any string-like argument, so `std::string`, `std::wstring`,
 // `absl::string_view`, `const char*`, and `const wchar_t*` are all accepted.
 // Likewise, `%d` accepts any integer-like argument, etc.
-
+//
+// Note: Compile-time format string checking is supported on GCC and
+// Clang. On MSVC, these checks are performed at runtime instead.
 template <typename... Args>
 using FormatSpec = str_format_internal::FormatSpecTemplate<
     str_format_internal::ArgumentToConv<Args>()...>;