Add qbittorrent specific section of known issues (#1591)

mkdocs/docs/known_issues_bugs.md

@@ -122,6 +122,11 @@ mergerfs before v2.41.0.) In newer versions of mergerfs and Linux
 When `mmap` is needed but not available you may see `ENODEV` or `No
 such device` errors though it will depend on the particular software.
 
+Details on enabling `mmap` can be found at:
+
+* [QuickStart](quickstart.md#configuration)
+* [Config > cache.files](config/cache.md#cachefiles)
+
 That said it is recommended that config and runtime files be stored on
 SSDs on a regular filesystem for performance reasons. See [What should
 mergerfs NOT be used
@@ -137,6 +142,25 @@ more compatible but could also be more performant in certain
 situations.
 
 
+### qbittorrent
+
+Same `mmap` concerns as mentioned above but called out specifically
+due to regularity of queries.
+
+The default "Disk IO type" setting is "Memory mapped files" which
+means it requires `mmap` support.
+
+If qbittorrent errors when using mergerfs ensure that either "Disk IO
+type" is set to "POSIX-compliant" or "Simple pread/pwrite"; or setup
+mergerfs to support `mmap`.
+
+References:
+
+* [Software using mmap](#software-using-mmap)
+* [QuickStart](quickstart.md#configuration)
+* [Config > cache.files](config/cache.md#cachefiles)
+
+
 ### rsync
 
 This isn't specific to mergerfs but when using `--sparse, -S` option