BUG/MEDIUM: doc: Fix replace-path action description

BUG/MEDIUM: doc: Fix replace-path action description

The description of the replace-path action does not reflect what the code
do. When the request path is replaced, the query-string is preserved. But the
documentation stated the query-string is part of the replacement, if any is
present. Most of time, when the doc and the code differ, the code is fixed. But
here, the replace-path action is pretty confusing because the set-path action is
only applied on the path. The query-string is left intact. And the path sample
fetch also ignores the query-string. In addition, the replace-path action is
quite recent. It was added in the 2.2. Thus, exceptionally, the documentation is
fixed instead.

Note that set-pathq and replace-pathq actions and pathq sample fetch will be
added to manipulate the path with the query-string.

This patch must be backported as far as 2.0.