Split an info file into an indirect file plus bounded-size subfiles.
Each subfile will be up to the number of characters that
`Info-split-threshold' specifies, plus one node.
To use this command, first visit a large Info file that has a tag
table. The buffer is modified into a (small) indirect info file which
should be saved in place of the original visited file.
The subfiles are written in the same directory the original file is
in, with names generated by appending `-' and a number to the original
file name. The indirect file still functions as an Info file, but it
contains just the tag table and a directory of subfiles. (fn)