[stringmapflag] Replace word 'tag' with 'k=v pair'.
[stringmapflag] Replace word 'tag' with 'k=v pair'.

stringmapflag is not necessarily used to set tags, it can be used to set
arbitrary k=v pairs.

R=tandrii@chromium.org

Change-Id: I4887fd9d7eb33cef50471f7b579f49384bf5527d
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1875427
Auto-Submit: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go