[LyX/master] Expand comment

[Juergen Spitzmueller]

commit e8c1cb5074e36a344553d57537f8177345a86686
Author: Juergen Spitzmueller <sp...@lyx.org>
Date:   Sun Dec 30 13:24:51 2018 +0100

    Expand comment
---
 src/Buffer.cpp |   11 +++++++++--
 1 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/src/Buffer.cpp b/src/Buffer.cpp
index 50cc2f2..54a4dff 100644
--- a/src/Buffer.cpp
+++ b/src/Buffer.cpp
@@ -2497,8 +2497,15 @@ void Buffer::checkIfBibInfoCacheIsValid() const
                return;
        }
 
-       // if we already know the cache is invalid, no need to check
-       // the timestamps
+       // If we already know the cache is invalid, stop here.
+       // This is important in the case when the bibliography
+       // environment (rather than Bib[la]TeX) is used.
+       // In that case, the timestamp check below gives no
+       // sensible result. Rather than that, the cache will
+       // be invalidated explicitly via invalidateBibInfoCache()
+       // by the Bibitem inset.
+       // Same applies for bib encoding changes, which trigger
+       // invalidateBibInfoCache() by InsetBibtex.
        if (!d->bibinfo_cache_valid_)
                return;