[PATCH v2] pretty-options.txt: describe supported encoding

["Krzysztof Żelechowski" <giecrilj@stegny.2a.pl>]

git log recognises only system encodings supported by iconv(1), but not 
POSIX character maps used by iconv(1p). Document it.

Signed-off-by:  <ne01026@shark.2a.pl>

diff --git a/Documentation/pretty-options.txt b/Documentation/pretty-
options.txt
index 27ddaf84a19..4f8376d681b 100644
--- a/Documentation/pretty-options.txt
+++ b/Documentation/pretty-options.txt
@@ -36,9 +36,13 @@ people using 80-column terminals.
        The commit objects record the encoding used for the log message
        in their encoding header; this option can be used to tell the
        command to re-code the commit log message in the encoding
-       preferred by the user.  For non plumbing commands this
-       defaults to UTF-8. Note that if an object claims to be encoded
-       in `X` and we are outputting in `X`, we will output the object
+       preferred by the user.
+       The encoding must be a system encoding supported by iconv(1),
+       otherwise this option will be ignored.
+       POSIX character maps used by iconv(1p) are not supported.
+       For non-plumbing commands this defaults to UTF-8.
+       Note that if an object claims to be encoded in `X`
+       and we are outputting in `X`, we shall output the object
        verbatim; this means that invalid sequences in the original
        commit may be copied to the output.