fix #2218 and three more report interval start dates

[simonmichael]

• ;doc: date adjustments, period headings: rewrite, clarify ["every Nth day" periodic rule violating start date #2218]
• ;doc: relnotes: correction to 1.29's date adjustment notes [imp: journal: periodic txns need not start on an interval boundary #1982], ["every Nth day" periodic rule violating start date #2218]
• fix: 'every Nth day of month from DATE' start date ["every Nth day" periodic rule violating start date #2218]
• fix: three more interval start date cases; add tests; cleanup ["every Nth day" periodic rule violating start date #2218]
• dev: rename some Intervals for clarity ["every Nth day" periodic rule violating start date #2218]

[simonmichael]

Here's the new docs and tests:

• https://hledger.org/dev/hledger.html#date-adjustments
• https://hledger.org/dev/hledger.html#period-headings
• https://github.com/simonmichael/hledger/blob/master/hledger/test/register/intervals.test#L140-L248