mention safety of concurrent access; wording improvements

README

@@ -14,11 +14,13 @@ currently Maildir and IMAP4 mailboxes are supported. New messages, message
 deletions and flag changes can be propagated both ways.
 ``mbsync'' is suitable for use in IMAP-disconnected mode.
 
-Synchronization is based on unique message identifiers (UIDs), so no
-identification conflicts can occur (as opposed to some other mail
+Synchronization is based on unique message identifiers (UIDs), so
+no identification conflicts can occur (unlike with some other mail
 synchronizers).
 Synchronization state is kept in one local text file per mailbox pair;
-multiple replicas of a mailbox can be maintained.
+these files are protected against concurrent ``mbsync'' processes.
+Mailboxes can be safely modified while ``mbsync'' operates.
+Multiple replicas of each mailbox can be maintained.
 
 isync is the project name, while mbsync is the current executable name; this
 change was necessary because of massive changes in the user interface. An

src/mbsync.1

@@ -35,11 +35,14 @@ New messages, message deletions and flag changes can be propagated both ways;
 the operation set can be selected in a fine-grained manner.
 .br
 Synchronization is based on unique message identifiers (UIDs), so no
-identification conflicts can occur (as opposed to some other mail synchronizers).
+identification conflicts can occur (unlike with some other mail synchronizers).
 OTOH, \fBmbsync\fR is susceptible to UID validity changes (that \fIshould\fR
 never happen, but see "Compatibility" in the README).
 Synchronization state is kept in one local text file per mailbox pair;
-multiple replicas of a mailbox can be maintained.
+these files are protected against concurrent \fBmbsync\fR processes.
+Mailboxes can be safely modified while \fBmbsync\fR operates
+(see \fBINHERENT PROBLEMS\fR below for a minor exception).
+Multiple replicas of each mailbox can be maintained.
 ..
 .SH OPTIONS
 .TP