Doc: gdalwarp: clarify that -t_srs might still be needed when using -ct (fixes #8242)

[rouault]

No description provided.

[bpurinton]

@rouault That does work, I tested it. But I guess now I'm confused, because I thought the ct proj pipeline is an alternative to using the t_srs; or, is it the case that the ct option would allow a different pipeline (if projinfo showed a few options), overriding the t_srs option?

I actually tested this by adding a slight y-offset in the proj pipeline (while still including the t_srs). Indeed, it seems that the ct options did override the standard t_srs usage.

In that case, perhaps my suggested change to the documentation is useful?

[bpurinton]

LGTM, thanks for the prompt help @rouault!