git-survey: announce the upcoming pivot into git repo structure

`git survey` started life as an experimental scale-measurement tool;
the preceding commits give `git repo structure` the path-level detail
tables and ref-scoping mechanism that were `git survey`'s main draw,
so the two now overlap substantially. Plan the migration explicitly:
add a short notice at the top of the description making clear which
of `git survey`'s knobs map to which `git repo structure` option, and
state that a future release will turn `git survey` into a thin shim
over `git repo structure`.

Putting the notice in the description (rather than only the synopsis)
ensures it shows up in `git help survey` rendering before the reader
sees any option specifics, so an operator skimming the page learns
about the replacement before adopting any survey-specific flags.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Documentation/git-survey.adoc

@@ -3,16 +3,23 @@ git-survey(1)
 
 NAME
 ----
-git-survey - EXPERIMENTAL: Measure various repository dimensions of scale
+git-survey - DEPRECATED: Measure various repository dimensions of scale
 
 SYNOPSIS
 --------
 [verse]
-(EXPERIMENTAL!) 'git survey' <options>
+(DEPRECATED!) 'git survey' <options>
 
 DESCRIPTION
 -----------
 
+NOTE: `git survey` is being superseded by `git repo structure`. New
+deployments and new features should use `git repo structure`; its
+`--ref-filter=<pattern>` option subsumes the various `--branches`,
+`--tags`, and `--remotes` flags here, and `--top=<N>` provides the
+same detail tables. A future release will turn `git survey` into a
+thin shim over `git repo structure`. See linkgit:git-repo[1].
+
 Survey the repository and measure various dimensions of scale.
 
 As repositories grow to "monorepo" size, certain data shapes can cause