[Docs] Explain incomplete dates in range queries (#30689)
[Docs] Explain incomplete dates in range queries (#30689)

The current documentation isn't very clear about how incomplete dates are
treated when specifying custom formats in a `range` query. This change adds a
note explaining how missing month or year coordinates translate to dates that
have the missings slots filled with unix time start date (1970-01-01)

Closes #30634
GlenRSmith/elasticsearch,HonzaKral/elasticsearch,coding0011/elasticsearch,gfyoung/elasticsearch,GlenRSmith/elasticsearch,uschindler/elasticsearch,coding0011/elasticsearch,nknize/elasticsearch,robin13/elasticsearch,uschindler/elasticsearch,coding0011/elasticsearch,scorpionvicky/elasticsearch,gfyoung/elasticsearch,HonzaKral/elasticsearch,gingerwizard/elasticsearch,robin13/elasticsearch,GlenRSmith/elasticsearch,uschindler/elasticsearch,robin13/elasticsearch,HonzaKral/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,gfyoung/elasticsearch,HonzaKral/elasticsearch,robin13/elasticsearch,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,robin13/elasticsearch,nknize/elasticsearch,scorpionvicky/elasticsearch,scorpionvicky/elasticsearch,GlenRSmith/elasticsearch,gingerwizard/elasticsearch,gfyoung/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,coding0011/elasticsearch,gfyoung/elasticsearch,uschindler/elasticsearch,coding0011/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch