[DOCS] Add info on calendar v fixed interval. Closes elastic#29410

q!

docs/reference/aggregations/bucket/datehistogram-aggregation.asciidoc

@@ -53,7 +53,7 @@ All minutes begin at 00 seconds.
 seconds of the following minute in the specified timezone, compensating for any
 intervening leap seconds, so that the number of minutes and seconds past the 
 hour is the same at the start and end. 
-* Multiple minutes (_n_m) are intervals of exactly 60x1000=60,000 milliseconds
+* Multiple minutes (__n__m) are intervals of exactly 60x1000=60,000 milliseconds
 each.
 
 hours (h) ::
@@ -63,7 +63,7 @@ All hours begin at 00 minutes and 00 seconds.
 minutes of the following hour in the specified timezone, compensating for any
 intervening leap seconds, so that the number of minutes and seconds past the hour
 is the same at the start and end. 
-* Multiple hours (_n_h) are intervals of exactly 60x60x1000=3,600,000 milliseconds
+* Multiple hours (__n__h) are intervals of exactly 60x60x1000=3,600,000 milliseconds
 each.
 
 days (d) ::
@@ -74,7 +74,7 @@ All days begin at the earliest possible time, which is usually 00:00:00
 of the following day in the specified timezone, compensating for any intervening
 time changes, so that the number of hours, minutes, and seconds of the day is
 the same at the start and end.
-* Multiple days (_n_d) are intervals of exactly 24x60x60x1000=86,400,000
+* Multiple days (__n__d) are intervals of exactly 24x60x60x1000=86,400,000
 milliseconds each.
 
 weeks (w) ::
@@ -83,34 +83,34 @@ weeks (w) ::
 and the same day of the week and time of the following week in the specified 
 timezone, so that
 the day of the week and time of day are the same at the start and end.
-* Multiple weeks (_n_w) are not supported.
+* Multiple weeks (__n__w) are not supported.
 
 months (m) ::
 
 * One month (1m) is the interval between the start day of the month and time of
 day and the same day of the month and time of the following month in the specified
 timezone, so that the day of the month and time of day are the same at the start
 and end.
-* Multiple months (_n_m) are not supported.
+* Multiple months (__n__m) are not supported.
 
 quarters (q) ::
 
 * One quarter (1q) is the interval between the start day of the month and time
 time of day and the same day of the month and time of day three months later,
 so that the day of the month and time of day are the same at the start and end. +
-* Multiple quarters (_n_q) are not supported.
+* Multiple quarters (__n__q) are not supported.
 
 years (y) ::
 
 * One year (1y) is the interval between the start day of the month and time of 
 day and the same day of the month and time of day the following year in the 
 specified timezone, so that the date and time are the same at the start and end. +
-* Multiple years (_n_y) are not supported.
+* Multiple years (__n__y) are not supported.
 
 Widely distributed applications must also consider vagaries such as countries that
 start and stop daylight savings time at 12:01 A.M., so end up with one minute of 
 Sunday followed by an additional 59 minutes of Saturday every fall, and countries
-that decide at whim to move across the international date line. Situations like
+that decide to move across the international date line. Situations like
 that can make irregular timezone offsets seem easy. 
 
 As always, rigorous testing will ensure that your time interval specification is