pgsql: Revise GIN README

Revise GIN README

We find GIN concurrency bugs from time to time. One of the problems here is
that concurrency of GIN isn't well-documented in README. So, it might be even
hard to distinguish design bugs from implementation bugs.

This commit revised concurrency section in GIN README providing more details.
Some examples are illustrated in ASCII art.

Also, this commit add the explanation of how is tuple layout in internal GIN
B-tree page different in comparison with nbtree.

Discussion: https://postgr.es/m/CAPpHfduXR_ywyaVN4%2BOYEGaw%3DcPLzWX6RxYLBncKw8de9vOkqw%40mail.gmail.com
Author: Alexander Korotkov
Reviewed-by: Peter Geoghegan
Backpatch-through: 9.4

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8165384babd96258ed521c4ea2567c70a1849948

Modified Files
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src/backend/access/gin/README | 209 +++++++++++++++++++++++++++++++++---------
1 file changed, 168 insertions(+), 41 deletions(-)