update documentation

Remove the "only works on eMMCs" caveats and describe the new
non-seekable target behavior.

README.md

@@ -37,9 +37,6 @@ implementation lacks:
 
 casync-nano is stable and is actively being used in the field.
 
-Currently, casync-nano only supports reading from and writing to block devices
-(such as eMMC storage).
-
 ## Dependencies
   * libcurl
   * openssl

doc/csn.1

@@ -55,13 +55,18 @@ Specifies the .\&caibx file that describes the image to be extracted.\&
 \fITARGET\fR
 .RS 4
 Specifies where the extracted image will be written to.\& This will normally
-be a block device.\& casync-nano currently does not support writing to MTD
-devices.\&
+be a block device.\& casync-nano can write to non-readable/non-seekable
+targets (such as an UBI volume after UBI_IOCVOLUP has been called) but in
+those cases, the target cannot be used as a store and casync-nano also
+cannot resume a previous partial update and will start from the beginning.\&
 .PP
 As a safety mechanism, casync-nano will refuse to create \fITARGET\fR when it
 does not exist.\& To extract an image to a regular file, create an empty file
 first.\&
 .PP
+Using "-" as the target will cause casync-nano to write the image to
+standard out.\&
+.PP
 .RE
 \fISTORE\fR
 .RS 4

doc/csn.1.scd

@@ -38,13 +38,18 @@ _INDEX_
 
 _TARGET_
 	Specifies where the extracted image will be written to. This will normally
-	be a block device. casync-nano currently does not support writing to MTD
-	devices.
+	be a block device. casync-nano can write to non-readable/non-seekable
+	targets (such as an UBI volume after UBI_IOCVOLUP has been called) but in
+	those cases, the target cannot be used as a store and casync-nano also
+	cannot resume a previous partial update and will start from the beginning.
 
 	As a safety mechanism, casync-nano will refuse to create _TARGET_ when it
 	does not exist. To extract an image to a regular file, create an empty file
 	first.
 
+	Using "-" as the target will cause casync-nano to write the image to
+	standard out.
+
 _STORE_
 	A generic source of chunks. This can either be a HTTP(S) URL pointing
 	towards a casync-style chunk store or a path to a local file or block device