pgsql: Add comment on why pulling data from a "name" index column can't

Add comment on why pulling data from a "name" index column can't crash.

It's been bothering me for several days that pretending that the cstring
data stored in a btree name_ops column is really a "name" Datum could lead
to reading past the end of memory. However, given the current memory
layout used for index-only scans in the btree code, a crash is in fact not
possible. Document that so we don't break it. I have not thought of any
other solutions that aren't fairly ugly too, and most of them lose the
functionality of index-only scans on name columns altogether, so this seems
like the way to go.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/8c8ba6d11b06e5a8b9fe5653a1cd17c437af5f7b

Modified Files
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src/backend/access/nbtree/nbtree.c | 10 ++++++++++
1 files changed, 10 insertions(+), 0 deletions(-)