reftex-index-phrases-wrap-long-lines

reftex-index-phrases-wrap-long-lines is a variable defined in `reftex-vars.el'.
Its value is

nil

Documentation:
Non-nil means, when indexing from the phrases buffer, wrap lines.
Inserting indexing commands in a line makes the line longer - often
so long that it does not fit onto the screen. When this variable is
non-nil, newlines will be added as necessary before and/or after the
indexing command to keep lines short. However, the matched text
phrase and its index command will always end up on a single line.

You can customize this variable.