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C comments: improve description of GiST NSN and GistBuildLSN

GiST indexes are complex, so adding more details in the code might helpsomeone.Discussion:https://postgr.es/m/20210302164021.GA364@momjian.us

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`@@ -10,6 +10,7 @@ GiST stands for Generalized Search Tree. It was introduced in the seminal paper`
`10`	`10`	`Jeffrey F. Naughton, Avi Pfeffer:`
`11`	`11`
`12`	`12`	`http://www.sai.msu.su/~megera/postgres/gist/papers/gist.ps`
	`13`	`+ https://dsf.berkeley.edu/papers/sigmod97-gist.pdf`
`13`	`14`
`14`	`15`	`and implemented by J. Hellerstein and P. Aoki in an early version of`
`15`	`16`	`PostgreSQL (more details are available from The GiST Indexing Project`

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`@@ -51,13 +51,20 @@`
`51`	`51`	`#defineF_HAS_GARBAGE(1 << 4)/* some tuples on the page are dead,`
`52`	`52`	`* but not deleted yet */`
`53`	`53`
`54`		`-/* NSN - node sequence number, a special-purpose LSN */`
	`54`	`+/*`
	`55`	`+ * NSN (node sequence number) is a special-purpose LSN which is stored on each`
	`56`	`+ * index page in GISTPageOpaqueData and updated only during page splits. By`
	`57`	`+ * recording the parent's LSN in GISTSearchItem.parentlsn, it is possible to`
	`58`	`+ * detect concurrent child page splits by checking if parentlsn < child's NSN,`
	`59`	`+ * and handle them properly. The child page's LSN is insufficient for this`
	`60`	`+ * purpose since it is updated for every page change.`
	`61`	`+ */`
`55`	`62`	`typedefXLogRecPtrGistNSN;`
`56`	`63`
`57`	`64`	`/*`
`58`		`- * Abogus LSN / NSN value used during indexbuild. Must be smaller than any`
`59`		`- * real or fake unlogged LSN, so thatafteran index buildfinishes, all the`
`60`		`- * splits are consideredcompleted.`
	`65`	`+ * Afake LSN / NSN value used during indexbuilds. Must be smaller than any`
	`66`	`+ * real or fake(unlogged) LSN generatedafterthe index buildcompletes so`
	`67`	`+ *that allsplits are consideredcomplete.`
`61`	`68`	`*/`
`62`	`69`	`#defineGistBuildLSN((XLogRecPtr) 1)`
`63`	`70`

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