config_dvr.html: Updated description of caching scheme and attached more up-to-date...

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diff --git a/docs/html/config_dvr.html b/docs/html/config_dvr.html
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--- a/docs/html/config_dvr.html
+++ b/docs/html/config_dvr.html
@@ -20,21 +20,26 @@
   <dd>Select the container format used to store recordings.
 
   <dt>Cache scheme
-  <dd>Select the cache scheme used to store recordings.
-  
+  <dd>Select the cache scheme used to store recordings. Leave as "system" unless you have a special case for one of the others.
+  <br><br>  
+  <dd>Whenever you read or write data to the filesystems, the information is kept (cached) in memory for a while. This means that regularly-access files are available quickly without going back to the disc; it also means that there's a disconnect when writing between the write request (from the application) and the actual write itself (to the disc/storage) as changes are buffered to be written in one go.</dd>
+
     <dl>
 
-      <dt>System
-      <dd>Standard system caching.
-      
-      <dt>Do not keep
-      <dd>Do not keep the stored data in system's cache.
-      
-      <dt>Sync
-      <dd>Sync the stored data with medium (disk).
-      
-      <dt>Sync + Do not keep
-      <dd>Combination of two above variants.
+      <dt>Unknown</dt>
+      <dd>A placeholder status, meaning that the configuration isn't properly set.</dd>
+
+      <dt>System</dt>
+      <dd>Change nothing and rely on standard (default) system caching to behave as it normally would.</dd>
+
+      <dt>Do not keep</dt>
+      <dd>Tell the system that you're not expecting to re-use the data soon, so don't keep it in cache. The data will still be buffered for writing. <i>Useful e.g. in a RAM-limited system like a Pi (given that you're unlikely to be watching while recording, so data can be discarded now and read back from disc later).</i></dd>
+
+      <dt>Sync</dt>
+      <dd>Tell the system to write the data immediately. This doesn't affect whether or not it's cached. <i>Useful e.g. if you've a particular problem with data loss due to delayed write (such as if you get frequent transient power problems).</i></dd>
+
+      <dt>Sync + Do not keep</dt>
+      <dd>A combination of last two variants above - data is written immediately and then discarded from cache.</dd>
 
     </dl>