[doc] add paragraph about 'fs' subsystem to sysfs.txt

Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>

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Documentation/filesystems/sysfs.txt
··· 246 devices/ 247 firmware/ 248 net/ 249 250 devices/ contains a filesystem representation of the device tree. It maps 251 directly to the internal kernel device tree, which is a hierarchy of ··· 264 drivers/ contains a directory for each device driver that is loaded 265 for devices on that particular bus (this assumes that drivers do not 266 span multiple bus types). 267 268 269 More information can driver-model specific features can be found in
··· 246 devices/ 247 firmware/ 248 net/ 249 + fs/ 250 251 devices/ contains a filesystem representation of the device tree. It maps 252 directly to the internal kernel device tree, which is a hierarchy of ··· 263 drivers/ contains a directory for each device driver that is loaded 264 for devices on that particular bus (this assumes that drivers do not 265 span multiple bus types). 266 + 267 + fs/ contains a directory for some filesystems. Currently each 268 + filesystem wanting to export attributes must create its own hierarchy 269 + below fs/ (see ./fuse.txt for an example). 270 271 272 More information can driver-model specific features can be found in