Add absolute path restriction to `svn-all-fast-export` command documentation

[ShadowCreator250]

The command documentation in the Ubuntu Manpage states for the Path to subversion repository:

Path to subversion repository is a backup copy of the subversion repository. Note that an svn url will not work due to the undistributed nature of svn.

I've noticed that

./svn2git/svn-all-fast-export --identity-map ./account-mapping.txt --rules ./rules.txt --debug-rules ./svn-local-repo/

produces this output:

Loading rules from: "./rules.txt"
Loading rules from "./rules.txt"
marksfile  "repo/marks-repo"
svn-all-fast-export: /build/subversion-owKwd0/subversion-1.13.0/subversion/libsvn_subr/dirent_uri.c:1011: svn_dirent_join: Assertion `svn_dirent_is_canonical(base, pool)' failed.
Aborted (core dumped)

but

./svn2git/svn-all-fast-export --identity-map ./account-mapping.txt --rules ./rules.txt --debug-rules /d/<path to local svn repo>/

works.

I feel like this should be noted in the command documentation.