Document that cache cleaning intervals < 1h are not supported

The documentation stated that time intervals in a second or minute range
could be used to define the interval, in which the web session cache
would be cleaned. In reality all time intervals were rounded to hours.
In the case of `5m` the interval was rounded to `0`, which caused an
error, when loading the plugin.

This change corrects the documentation. A different approach would be to
allow smaller time intervals, but since cleaning the cache in too small
intervals would in reality not make sense, this was not done.

Change-Id: Id5b76d8a89e6e2d9b777a6a5b7206540f98ca74b

src/main/resources/Documentation/config.md

diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index cc5e71f..e0224e5 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -34,8 +34,6 @@
 
 Values should use common time unit suffixes to express their setting:
 
-* s, sec, second, seconds
-* m, min, minute, minutes
 * h, hr, hour, hours
 * d, day, days
 * w, week, weeks (`1 week` is treated as `7 days`)
@@ -44,6 +42,8 @@
 
 If a time unit suffix is not specified, `hours` is assumed.
 
+Time intervals smaller than one hour are not supported.
+
 If 'cleanupInterval' is not present in the configuration, the
 cleanup operation is triggered every 24 hours.