Update `-ddump-minimal-imports` entry in user's guide
Update `-ddump-minimal-imports` entry in user's guide

The entry is updated to mention explicitly the feature implemented
for #7957, as well as rewording and improving the docbook markup.

Signed-off-by: Herbert Valerio Riedel <202fbcf3c00694524024feb79f3bb8fc1c4e4f9c@gnu.org>

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