docs: Mention that #pragma once could be problematic with PCH

Fixes #1615.

diff --git a/doc/MANUAL.adoc b/doc/MANUAL.adoc
index e67b3589e219f180020fdd63e4d89f3b8db18b7b..b8f48a6fe0ea9cb91c83a4c3e3b79ec077d4ace7 100644 (file)
--- a/doc/MANUAL.adoc
+++ b/doc/MANUAL.adoc
@@ -1802,7 +1802,7 @@ directories:
 
 == Precompiled headers
 
-Ccache has support for precompiled headers with GCC and Clang. However, you have
+Ccache has limited support for precompiled headers with GCC and Clang. You have
 to do some things to make it work properly:
 
 * You must set <<config_sloppiness,*sloppiness*>> to *pch_defines,time_macros*.
@@ -1833,6 +1833,9 @@ be used (if available) or ccache will fall back to running the real compiler and
 increase the statistics counter "`Preprocessing failed`" (if the non-precompiled
 header file is not available).
 
+Also, you might get into trouble if header files use `#pragma once` instead of
+include guards.
+
 
 == C++ modules