In compat 10 and earlier, dh_install(1) may be a better tool for handling the upstream documentation, when upstream's own build system installs all the desired documentation correctly. In this case, dh_installdocs is still useful for installing packaging related documentation (e.g. the debian/copyright file).
From debhelper compatibility level 11 on, dh_install will fall back to looking in debian/tmp for files, if it does not find them in the current directory (or wherever you've told it to look using --sourcedir).
In compat 11 and later, dh_installdocs offers many of the features that dh_install(1) also has. Furthermore, dh_installdocs also supports the nodoc build profile to exclude documentation (regardless of compat level).
Note that README.debian files are also installed as README.Debian, and TODO files will be installed as TODO.Debian in non-native packages.
Note that this is not the same as the --sourcedirectory option used by the dh_auto_* commands. You rarely need to use this option, since dh_installman automatically looks for files in debian/tmp in debhelper compatibility level 11 and above.
In compat 11 (or later), this option is only useful if debhelper's auto-detection of the main package is wrong. The option can also be used to silence a warning from debhelper when the auto-detection fails but the default happens to be correct.
This option cannot be used when dh_installdocs is instructed to act on multiple packages. If you need this option, you will generally need to combine it with -p to ensure exactly one package is acted on.
Please keep in mind that some documentation (the copyright file, README.Debian, etc.) will be unaffected by this option.
debhelper will try to avoid installing files into linked documentation directories that would cause conflicts with the linked package. The -A option will have no effect on packages with linked documentation directories, and copyright, changelog, README.Debian, and TODO files will not be installed.
(An older method to accomplish the same thing, which is still supported, is to make the documentation directory of a package be a dangling symlink, before calling dh_installdocs.)
Please note that this option only applies to the documentation directory for the package itself. When the package ships documentation for another package (e.g. see --doc-main-package), it will not use a symlink for the documentation of the other package.
CAVEAT 1: If a previous version of the package was built without this option and is now built with it (or vice-versa), it requires a ``dir to symlink'' (or ``symlink to dir'') migration. Since debhelper has no knowledge of previous versions, you have to enable this migration itself.
This can be done by providing a "debian/package.maintscript" file and using dh_installdeb(1) to provide the relevant maintainer script snippets.
CAVEAT 2: The use of --link-doc should only be done when the packages have same ``architecture'' type. A link from an architecture independent package to an architecture dependent package (or vice versa) will not work. Since compat 10, debhelper will actively reject unsupported combinations.
README TODO debian/notes-for-maintainers.txt docs/manual.txt docs/manual.pdf docs/manual-html/